code: def register_on_additions_state_changed(self, callback):
          
          event_type = library.VBoxEventType.on_additions_state_change
          return self.event_source.register_callback(callback, event_type)
docs: Set the callback function to consume on additions state changed
          events.
  
          Callback receives a IAdditionsStateChangedEvent object.
  
          Note: Interested callees should query IGuest attributes to find out
                what has changed.
  
          Returns the callback_id