Check for system bus before using DBus
Check for system bus before using DBus

If there's no system bus, there's no need to check for Connman or
NetworkManager availability.

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=727709

GNOME/gupnp,GNOME/gupnp,GNOME/gupnp,GNOME/gupnp