How to use the aexpect.Tail function in aexpect

self.vhost_threads = re.findall("\w+\s+(\d+)\s.*\[vhost-%s\]" %
                                            self.get_pid(), o)

            # Establish a session with the serial console -- requires a version
            # of netcat that supports -U
            self.serial_console = aexpect.ShellSession(
                "nc -U %s" % self.get_serial_console_filename(),
                auto_close=False,
                output_func=utils_misc.log_line,
                output_params=("serial-%s.log" % name,),
                prompt=self.params.get("shell_prompt", "[\#\$]"))

            for key, value in self.logs.items():
                outfile = "%s-%s.log" % (key, name)
                logging.info("add log: %s" % outfile)
                self.logsessions[key] = aexpect.Tail(
                    "nc -U %s" % value,
                    auto_close=False,
                    output_func=utils_misc.log_line,
                    output_params=(outfile,))

            # start guest
            if self.monitor.verify_status("paused"):
                try:
                    self.monitor.cmd("cont")
                except kvm_monitor.QMPCmdError, e:
                    if ((e.data['class'] == "MigrationExpected") and
                        (migration_mode is not None)):
                        logging.debug("Migration did not start yet...")
                    else:
                        raise e

def start(self, arg_str='', wait_for_working=True):
        """
        Start libvirtd session.

        :param arg_str: Argument passing to the session
        :param wait_for_working: Whether wait for libvirtd finish loading
        """
        if self.gdb:
            self.gdb.run(arg_str=arg_str)
            self.pid = self.gdb.pid
        else:
            self.tail = aexpect.Tail(
                "%s %s" % (LIBVIRTD, arg_str),
                output_func=self._output_handler,
                termination_func=self._termination_handler,
            )
            self.running = True

        if wait_for_working:
            self.wait_for_working()

tmp_serial = self.serial_ports[0]
            except IndexError:
                raise virt_vm.VMConfigMissingError(name, "isa_serial")

            self.serial_console = aexpect.ShellSession(
                "nc -U %s" % self.get_serial_console_filename(tmp_serial),
                auto_close=False,
                output_func=utils_misc.log_line,
                output_params=("serial-%s-%s.log" % (tmp_serial, name),),
                prompt=self.params.get("shell_prompt", "[\#\$]"))
            del tmp_serial

            for key, value in self.logs.items():
                outfile = "%s-%s.log" % (key, name)
                logging.debug("Add log: %s" % outfile)
                self.logsessions[key] = aexpect.Tail(
                    "nc -U %s" % value,
                    auto_close=False,
                    output_func=utils_misc.log_line,
                    output_params=(outfile,))

            # start guest
            if self.monitor.verify_status("paused"):
                try:
                    self.monitor.cmd("cont")
                except kvm_monitor.QMPCmdError, e:
                    if ((e.data['class'] == "MigrationExpected") and
                        (migration_mode is not None)):
                        logging.debug("Migration did not start yet...")
                    else:
                        raise e

try:
                tmp_serial = self.serial_ports[0]
            except IndexError:
                raise virt_vm.VMConfigMissingError(name, "isa_serial")

            self.serial_console = aexpect.ShellSession(
                "nc -U %s" % self.get_serial_console_filename(tmp_serial),
                auto_close=False,
                output_func=utils_misc.log_line,
                output_params=("serial-%s-%s.log" % (tmp_serial, name),),
                prompt=self.params.get("shell_prompt", "[\#\$]"))
            del tmp_serial

            for key, value in self.logs.items():
                outfile = "%s-%s.log" % (key, name)
                self.logsessions[key] = aexpect.Tail(
                    "nc -U %s" % value,
                    auto_close=False,
                    output_func=utils_misc.log_line,
                    output_params=(outfile,))
                self.logsessions[key].set_log_file(outfile)

            if params.get("paused_after_start_vm") != "yes":
                # start guest
                if self.monitor.verify_status("paused"):
                    try:
                        self.monitor.cmd("cont")
                    except qemu_monitor.QMPCmdError, e:
                        if ((e.data['class'] == "MigrationExpected") and
                            (migration_mode is not None)):
                            logging.debug("Migration did not start yet...")
                        else:

if "tcpdump" not in env and params.get("run_tcpdump", "yes") == "yes":
        cmd = "%s -npvi any 'dst port 68'" % utils_misc.find_command("tcpdump")
        if params.get("remote_preprocess") == "yes":
            logging.debug("Starting tcpdump '%s' on remote host", cmd)
            login_cmd = ("ssh -o UserKnownHostsFile=/dev/null -o \
                         PreferredAuthentications=password -p %s %s@%s" %
                         (port, username, address))
            env["tcpdump"] = aexpect.ShellSession(
                login_cmd,
                output_func=_update_address_cache,
                output_params=(env["address_cache"],))
            remote._remote_login(env["tcpdump"], username, password, prompt)
            env["tcpdump"].sendline(cmd)
        else:
            logging.debug("Starting tcpdump '%s' on local host", cmd)
            env["tcpdump"] = aexpect.Tail(
                command=cmd,
                output_func=_update_address_cache,
                output_params=(env["address_cache"],))

        if utils_misc.wait_for(lambda: not env["tcpdump"].is_alive(),
                              0.1, 0.1, 1.0):
            logging.warn("Could not start tcpdump")
            logging.warn("Status: %s" % env["tcpdump"].get_status())
            logging.warn("Output:" + utils_misc.format_str_for_message(
                env["tcpdump"].get_output()))

    # Destroy and remove VMs that are no longer needed in the environment
    requested_vms = params.objects("vms")
    for key in env.keys():
        vm = env[key]
        if not utils_misc.is_vm(vm):