network/net-device-ethernet: Disconnect from NMConnection signals

Otherwise we'll crash in our handler if the signal fires after we're
finalized i.e. user leaves the network panel.

https://bugzilla.gnome.org/show_bug.cgi?id=708468
network: Correctly unref NetDevice instances when needed

We have to take the GtkListStore own reference into account.

https://bugzilla.gnome.org/show_bug.cgi?id=708468