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.

Note: Some tests for isa_serial still have problem, I find they failed because qemu_vm module can't handle 2 isa serials console correctly, I'll fix them in separated pullreq then.