[nix] output vm packages for each host

this now runs any host as microvm. it removes shared directories for
microvms so no manuall setup is needed (expect you want persistence).
i took it from c3d2, thanks guys for the inspiration <3
https://gitea.c3d2.de/c3d2/nix-config/src/branch/master/packages.nix

outputs.nix

@@ -56,7 +56,46 @@ in (utils.lib.eachSystem (builtins.filter filter_system utils.lib.defaultSystems
           cp -r ./book/* $dest
         '';
       };
-    };
+    } //
+
+    builtins.foldl'
+    (result: host:
+      let
+        inherit (self.nixosConfigurations.${host}) config;
+      in
+      result // {
+        # boot any machine in a microvm
+        "${host}-vm" = (self.nixosConfigurations.${host}.extendModules {
+          modules = [{
+            microvm = {
+              mem = pkgs.lib.mkForce 4096;
+              hypervisor = pkgs.lib.mkForce "qemu";
+              socket = pkgs.lib.mkForce null;
+              shares = pkgs.lib.mkForce [{
+                tag = "ro-store";
+                source = "/nix/store";
+                mountPoint = "/nix/.ro-store";
+              }];
+              interfaces = pkgs.lib.mkForce [{
+                type = "user";
+                id = "eth0";
+                mac = "02:23:de:ad:be:ef";
+              }];
+            };
+            boot.isContainer = pkgs.lib.mkForce false;
+            users.users.root.password = "";
+            fileSystems."/".fsType = pkgs.lib.mkForce "tmpfs";
+            services.getty.helpLine = ''
+              Log in as "root" with an empty password.
+              Use "reboot" to shut qemu down.
+            '';
+          }] ++ pkgs.lib.optionals (! config ? microvm) [
+            microvm.nixosModules.microvm
+          ];
+        }).config.microvm.declaredRunner;
+    })
+    { }
+    (builtins.attrNames self.nixosConfigurations);
 
     apps = {
       docs = {