code: def register_on_event_source_changed(self, callback):
          
          event_type = library.VBoxEventType.on_event_source_changed
          return self.event_source.register_callback(callback, event_type)
docs: Set the callback function to consume on event source changed
          events.  This occurs when a listener is added or removed.
  
          Callback receives a IEventStateChangedEvent object.
  
          Returns the callback_id