Qemu guest agent bugfix and updates

Hi, there,
This pullreq contains some updates for guest agent module and tests. 

This pullreq also tries to resolve a problem which guest_agent makes autotest hang when sending guest agent command to VM after VM is dead.

```
Qingtang Zhou (13):
  qemu.tests.cfg: Enable qemu_guest_agent tests on newer Fedora host
  qemu.tests: Fix module import problem in qemu_guest_agent_snapshot test
  qemu.tests.qemu_guest_agent: Update fsfreeze/fsthaw workflow
  virt.guest_agent: Remove qemu monitor operation from this module
  virt.guest_agent: Remove reference to vm object in QemuAgent class
  virt.guest_agent: Define constant for guest agent serial type
  virt.guest_agent: Remove VAgentNotSupportedSerialError
  qemu.tests.qemu_guest_agent: Update QemuAgent initiation code
  qemu.tests.qemu_guest_agent: Fix test error message
  virt.qemu_vm: Seperate qemu monitor creation code from vm.create()
  virt.qemu_monitor: Store qemu VM's name in monitor object
  virt.qemu_monitor: Add a vm_is_dead_func member for Monitor class
  virt.qemu_monitor: Check if monitor is dead before receiving msg from it

 qemu/tests/cfg/qemu_guest_agent.cfg     |  2 +-
 qemu/tests/qemu_guest_agent.py          | 23 +++++++---
 qemu/tests/qemu_guest_agent_snapshot.py |  2 +-
 virttest/guest_agent.py                 | 57 ++++++++++++-------------
 virttest/qemu_monitor.py                | 19 ++++++---
 virttest/qemu_vm.py                     | 74 ++++++++++++++++++---------------
 6 files changed, 99 insertions(+), 78 deletions(-)
```