When a virtual device is removed and the instance gets finalized we're
not disconnecting from the real NMDevice's notify signal and thus end
up crashing when the signal fires afterwards.
Avoid that by using g_signal_connect_object() so that when we're
finalized the disconnection happens automatically.
https://bugzilla.gnome.org/show_bug.cgi?id=749972
Bond, bridge, and VLAN devices may not actually exist until their
connections are brought up. So for those types, create device items
(of type NetVirtualDevice or a subclass) as soon as we see the
NMConnection, and then watch for the NMDevice being added later.
https://bugzilla.gnome.org/show_bug.cgi?id=677145
