Add a systemd generator to imperatively generate microvms.target

[SuperSandro2000]

see https://www.freedesktop.org/software/systemd/man/latest/systemd.generator.html,

microvm.nix/nixos-modules/host/default.nix

Line 263 in 80bddbd

wants = map (name: "microvm@${name}.service") config.microvm.autostart;

cc @marenz2569 @astro