Please fix use of gupnp_service_info_get_introspection

[phako]

gupnp_service_info_get_introspection is more or less broken when using GUPnP > 0.20.13 (which is from 2014). Please fix this todo:

    /* TODO: this weak_ref hack is needed as
       gupnp_service_info_get_introspection iterates the main loop.
       This can result in our device getting deleted before this
       function returns.  Ultimately, this code needs to be re-written
       to use gupnp_service_info_get_introspection_async but this cannot
       really be done until GUPnP provides a way to cancel this function. */

The requested function is available since GUPnP 0.20.9 (gupnp_service_info_get_instrospection_async_full, since Dec 2013).

I seem to get random lock-ups there. This might or might not be related to Issue #156

[phako]

Actually, that's exactly what issue #104 is about.