Revert "nixos/qemu-vm: replace postBootCommands by a systemd service" (#496623)

    # allow `system.build.toplevel' to be included.  (If we had a direct

    # reference to ${regInfo} here, then we would get a cyclic

    # dependency.)

    systemd.services.register-nix-paths = lib.mkIf config.nix.enable {

      wantedBy = [

        "multi-user.target"

      ];

      after = [

        "nix-daemon.service"

        "nix-daemon.socket"

      ];

      restartIfChanged = false;

      serviceConfig = {

        Type = "oneshot";

        RemainAfterExit = true;

      };

      script = ''

    boot.postBootCommands = lib.mkIf config.nix.enable ''

      if [[ "$(cat /proc/cmdline)" =~ regInfo=([^ ]*) ]]; then

          ${config.nix.package.out}/bin/nix-store --load-db < "''${BASH_REMATCH[1]}"

        ${config.nix.package.out}/bin/nix-store --load-db < ''${BASH_REMATCH[1]}

      fi

    '';

    };

    boot.initrd.availableKernelModules =

      optional (cfg.qemu.diskInterface == "scsi") "sym53c8xx" ++ optional (cfg.tpm.enable) "tpm_tis";