Finish up the tutorial with QEMU VM

[roberth]

Basically something like dev/example.nix in nixops4/nixops4@main...eval-sudo-wip#diff-229367db3333e23b9ffb8cf7233ff62cb7f595dddbb665caadd9178953e5f6e7
plus some writing

[KiaraGrouwstra]

a related concern may be usage in tests - see e.g. basic/cli tests in https://git.fediversity.eu/fediversity/fediversity/src/branch/main/deployment/check