virsh_reboot.py: bugfix for qemu-ga

* some old linux version doesn't support qemu-ga command,
  such as RHEL5.11.
* if there's a instance of qemu-guest-agent, executing the command
  will encounter the following error,

  output: '[root@localhost ~]#
  1419277122.516774: critical: Cannot lock pid file,
                               Resource temporarily unavailable\n
  1419277122.516788: critical: failed to create pidfile\n'

so it's better to do some check before qemu-ga is executed.
Merge pull request #400 from PandaWei/bugfix

virsh_reboot.py: bugfix for qemu-ga