stbt virtual-stb: Fix handling of "--docker-image" arg

This fixes an error introduced in recent refactorings related to
ssh-endpoint / service.
stbt virtual-stb: Expose lirc via IP, not Unix-domain socket

This way it's easier to run "stbt run" on the host against a virtual-stb
running in a docker container (as opposed to when "stbt run" is also
running inside a docker container, which worked because the 2 containers
shared the /run/lirc/lircd socket).