Variant of gupnp_service_action_get_value that creates a new GValue and
returns that instead of taking an already initialized GValue and setting
it. This is mainly done to allow PyGI to deal with service actions. PyGI
maintainer refuses to handle reference type 'in' parameters correctly
because the only way to do that is 'inefficient'.

Co-author and reviewer: Zeeshan Ali (Khattak) <zeeshanak@gnome.org>