GLib registers number->string value transforms (meaning that
number-valued properties like NMSettingVlan:id or NMSettingWired:mtu
get loaded into their NmtNewtEntries correctly), but not the
corresponding string->number transforms (meaning changes made in the
entries don't get propagated back to the settings, and due to
http://bugzilla.gnome.org/show_bug.cgi?id=726574, there's no warning
about this). Fix this by registering our own transforms.
