code: def register_on_snapshot_changed(self, callback):
          
          event_type = library.VBoxEventType.on_snapshot_changed
          return self.event_source.register_callback(callback, event_type)
docs: Set the callback function to consume on snapshot changed events
          which occur when snapshot properties have been changed.
  
          Callback receives a ISnapshotChangedEvent object.
  
          Returns the callback_id