nixos: Generalise the container tests in stage-2 boot

This way, stage-2 behaves correctly also for libvirt-lxc containers.

Some more discussion on this:
https://github.com/NixOS/nixpkgs/commit/a7a08188bf650ababa36300a9a6f34169e2a73bf
https://github.com/NixOS/nixpkgs/commit/bfe46a653ba2f8ff9902128f485cbd87c49cbca7