Issue description

systemd-nspawn containers started with --private-network (or related options) only see a loopback interface, the desired interface to the host is missing. Same applies to containers created by nixos-container. Problem exists with linux 4.14 and systemd 238.
With linux 4.9 and systemd 238, container networking looks ok.

See also discussion in 6b9771e4a

/cc @fpletz @vcunat

Steps to reproduce

Any of these:

• nix-build nixos/tests/containers-imperative.nix fails, container complains about missing host0 interface
• Try to start a container created by nixos-container create foo.
• manual tests with systemd-nspawn -n show only loopback interface in container.

Technical details

18.03pre131587.b6ddb9913f2.