virtual network interfaces: make systemd aware of the relation between device and service

If I want to bring down tap0.service (or systemd wants to do this
during a configuration-change which changes the path to tunctl),
openvpn (or other services using tap0) need to be brought down as
well, otherwise tunctl -d is not able to remove the tap0 device,
leaving it in a failed (but "up") state.