vms: add support for nix configured virtual machines

modules/default.nix

@@ -14,6 +14,7 @@
     ./storj.nix
     ./tailscale.nix
     ./users.nix
+    ./vms.nix
     ./www/global.nix
     ./www/www-repo.nix
   ];

modules/vms.nix

@@ -0,0 +1,21 @@
+{ config, lib, ... }:
+
+{
+  options = {
+    boot.isVirtualMachine = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = lib.mdDoc ''
+        Whether this NixOS machine is a virtual machine running in another NixOS system.
+      '';
+    };
+
+    virtualMachines = lib.mkOption {
+      type = lib.types.attrsOf
+        (lib.types.submodule (
+          { config, options, name, ... }:
+          { }
+        ))
+        };
+    };
+  }