enhance processWatchdogEvent()

enhance processWatchdogEvent()

This patch does the following two things:
1. hold an extra reference while handling watchdog event
   If the domain is not persistent, and qemu quits unexpectedly before
   calling processWatchdogEvent(), vm will be freed and the function
   processWatchdogEvent() will be dangerous.

2. unlock qemu driver and vm before returning from processWatchdogEvent()
   When the function processWatchdogEvent() failed, we only free wdEvent,
   but forget to unlock qemu driver and vm, free dumpfile.