Improve handling of server deletions.
 * Have the API return a server's state as DELETED when
   the deleted flag has been set in the VirtualMachine model.
   This makes the server immediately invisible, instead of waiting
   for an indefinite amount of time for the Ganeti backend to remove
   the VM.
 * Set the deleted flag in the VirtualMachine model when Ganeti reports
   successful removal of an instance. This catches instance deletions
   initiated by the admin at the Ganeti backend.