Asynchronously invokes the PmStandby D-Bus method on this.
When the operation is finished,
callback will be invoked in the thread-default main loop of the thread you are calling
this method from. You can then call call_pm_standby.end to get the result of the operation.
See call_pm_standby_sync for the synchronous, blocking version of this method.
Argument to pass with the method invocation.
A Cancellable or
A TaskReadyCallback to call when the request is satisfied or
User data to pass to