libxl_driver: Resolve Coverity errors

libxl_driver: Resolve Coverity errors

1. The virObjectLock() call was unconditional, but Unlock was conditional
   on vm being valid.  Removed the check

2. A call to virDomainEventNewFromObj() isn't guaranteed to return an
   event - that check needs to be made prior to libxlDomainEventQueue()
   of the event. Did not add libxlDriverLock/Unlock around the call since
   some callers already have lock taken

3. Need to initialize fd = -1 in libxlDoDomainSave() since we can jump
   to cleanup before it's set.

4. Missing break;'s in libxlDomainModifyDeviceFlags() for case
   LIBXL_DEVICE_UPDATE.  The default: case would report an error