Description of problem:

- run migration cancel using vdsClient on destination server where vm is on migrating-dest results an AttribureError: 

Thread-177::ERROR::2012-01-11 10:03:22,823::clientIF::90::vds::(wrapper) Traceback (most recent call last):
  File "/usr/share/vdsm/clientIF.py", line 80, in wrapper
    res = f(*args, **kwargs)
  File "/usr/share/vdsm/clientIF.py", line 415, in migrateCancel
    return vm.migrateCancel()
  File "/usr/share/vdsm/vm.py", line 1017, in migrateCancel
    self._migrationSourceThread.stop()
  File "/usr/share/vdsm/libvirtvm.py", line 454, in stop
    self._vm._dom.abortJob()
AttributeError: 'NoneType' object has no attribute 'abortJob


 449     def stop(self):
 450         # if its locks we are before the migrateToURI2()
 451         # call so no need to abortJob()
 452         try:
 453             self._migrationCanceledEvt = True
 454             self._vm._dom.abortJob()


we should check if object is not None before using it, if None, raise proper error.

 455         except libvirt.libvirtError, e:
 456             # TODO: yes its not nice searching in the error message,
 457             # but this is the libvrirt solution
 458             # this will be solved at bz #760149
 459             if e.get_error_code() == libvirt.VIR_ERR_OPERATION_FAILED and \
 460                     'canceled by client' in e.get_error_message():
 461                     # this is exception that libvirt raise when calling
 462                     # abortJob()
 463                     raise
 464             elif not self._preparingMigrationEvt:
 465                     raise