GUPnPService used to hold a reference on itself for the run-time of
the initial asynchronous introspection. Due to this reference the
object can't be disposed until this call has finished. Now if you
create a device with services and immidiately destroy it, the
service will still be alive holding a pointer to the XML device
description that is not any longer valid. This leads to a crash
when the asynchronous introspection finishes.

Fix this crash by not keeping a reference on ourselves. The pending
introspection is then cancelled from the dispose handler.