ova: add cloneConfigExtra option

Customize virtualbox ovas to contain a clone config option giving some
useful hints.

Fixes #38429

nixos/modules/installer/virtualbox-demo.nix

@@ -22,4 +22,42 @@ with lib;
 
   powerManagement.enable = false;
   system.stateVersion = mkDefault "18.03";
+
+  installer.cloneConfigExtra = ''
+  # Let demo build as a trusted user.
+  # nix.trustedUsers = [ "demo" ];
+
+  # Mount a VirtualBox shared folder.
+  # This is configurable in the VirtualBox menu at
+  # Machine / Settings / Shared Folders.
+  # fileSystems."/mnt" = {
+  #   fsType = "vboxsf";
+  #   device = "nameofdevicetomount";
+  #   options = [ "rw" ];
+  # };
+
+  # By default, the NixOS VirtualBox demo image includes SDDM and Plasma.
+  # If you prefer another desktop manager or display manager, you may want
+  # to disable the default.
+  # services.xserver.desktopManager.plasma5.enable = lib.mkForce false;
+  # services.xserver.displayManager.sddm.enable = lib.mkForce false;
+
+  # Enable GDM/GNOME by uncommenting above two lines and two lines below.
+  # services.xserver.displayManager.gdm.enable = true;
+  # services.xserver.desktopManager.gnome3.enable = true;
+
+  # Set your time zone.
+  # time.timeZone = "Europe/Amsterdam";
+
+  # List packages installed in system profile. To search, run:
+  # \$ nix search wget
+  # environment.systemPackages = with pkgs; [
+  #   wget vim
+  # ];
+
+  # Enable the OpenSSH daemon.
+  # services.openssh.enable = true;
+
+  system.stateVersion = mkDefault "18.03";
+  '';
 }