[UPnP] Handle GetServers() asynchronously
[UPnP] Handle GetServers() asynchronously

The call won't take long normally, and the app isn't really useful
until it returns. Nevertheless, D-Bus calls should still be
asynchronous.

Signed-off-by: Jussi Kukkonen <589e25658b1bfd3646cd0c60eea9e9594b5e0b5b@intel.com>

lferrandis/dleyna-control,markdryan/dleyna-control,rmerlino/dleyna-control,01org/dleyna-control,jku/dleyna-control