base/default.nix

@@ -7,6 +7,7 @@
 
     ./networking.nix
     ./nix.nix
+    ./vm.nix
 
     ./services/acme.nix
     ./services/auto-upgrade.nix
@@ -64,4 +65,3 @@
   # Trusted users on the nix builder machines
   users.groups."nix-builder-users".name = "nix-builder-users";
 }
-

base/vm.nix

@@ -0,0 +1,14 @@
+{ lib, ... }:
+
+# This enables
+#     lib.mkIf (!config.virtualisation.isVmVariant) { ... }
+
+{
+  options.virtualisation.isVmVariant = lib.mkOption {
+    type = lib.types.bool;
+    default = false;
+  };
+  config.virtualisation.vmVariant = {
+    virtualisation.isVmVariant = true;
+  };
+}