network: allow selecting a specific VPN or virtual interface

Overload the show-device operation to accept paths to NMConnections,
which can be matched to VPN connections or unadopted virtual
interfaces.
This will be used by gnome-shell to implement the VPN Settings
menu item.

https://bugzilla.gnome.org/show_bug.cgi?id=724547
network: handle argv after cold-plugging connections

To pick up show-device with a connection path (VPN or virtual),
we need to handle the arguments again after the connections are
read.

https://bugzilla.gnome.org/show_bug.cgi?id=724547