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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def register_on_machine_registered(self, callback):
 """Set the callback function to consume on machine registered events. Callback receives a IMachineRegisteredEvent object. Returns the callback_id """

        event_type = library.VBoxEventType.on_machine_registered
        return self.event_source.register_callback(callback, event_type)