Refactor testvm into reusable nixosModule for multiple architectures

This extracts the testvm configuration into a shared nixosModules.testvm
module that can be reused across different architectures. The module is
now used by both testvm-x86_64 and testvm-aarch64 nixosConfigurations,
eliminating code duplication.

Additionally, nixosConfigurations have been moved to the top-level flake
outputs to follow Nix conventions, rather than being nested inside
eachDefaultSystem. The virtwl proxy reference now uses ${pkgs.system}
instead of hardcoded x86_64-linux to support both architectures.

flake.nix

@@ -9,6 +9,40 @@
   };
 
   outputs = { self, nixpkgs, flake-utils, virtwl, ... }: {
+    nixosModules.testvm = { pkgs, ... }: {
+      system.stateVersion = "25.11";
+
+      fonts.packages = [ pkgs.adwaita-fonts pkgs.dejavu_fonts ];
+      environment.systemPackages = [
+        pkgs.fastfetch
+        pkgs.htop
+        virtwl.packages.${pkgs.system}.proxy
+        pkgs.wayland-utils
+        pkgs.weston
+        pkgs.waycheck
+        pkgs.vulkan-tools
+        pkgs.glmark2
+        pkgs.mesa-demos
+        pkgs.xorg.xeyes
+        pkgs.xterm
+        # pkgs.vkquake # build broken: Program 'spirv-remap' not found
+        pkgs.veloren
+        pkgs.kdePackages.kate
+        pkgs.adwaita-icon-theme
+        pkgs.amberol
+        pkgs.bustle
+        pkgs.d-spy
+        pkgs.gnome-text-editor
+        pkgs.firefox
+        pkgs.ffmpeg-full
+        pkgs.mpv
+        pkgs.libva-utils
+        pkgs.tailscale
+        pkgs.zerotierone
+        pkgs.localsend
+      ];
+    };
+
     nixosModules.default = { pkgs, lib, utils, config, ... }: let
       useTTY = {
         TTYPath = "/dev/hvc0";
@@ -217,6 +251,22 @@
       hardware.graphics.enable = true;
       hardware.graphics.package = self.packages.${pkgs.system}.mesa;
     };
+
+    nixosConfigurations.testvm-x86_64 = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        self.nixosModules.default
+        self.nixosModules.testvm
+      ];
+    };
+
+    nixosConfigurations.testvm-aarch64 = nixpkgs.lib.nixosSystem {
+      system = "aarch64-linux";
+      modules = [
+        self.nixosModules.default
+        self.nixosModules.testvm
+      ];
+    };
   } // flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = import nixpkgs {
@@ -311,45 +361,5 @@
           patches = old.patches ++ [ ./radvmmio.patch ]; # already merged to git
         });
 
-        nixosConfigurations.testvm = nixpkgs.lib.nixosSystem {
-          inherit system;
-          modules = [
-            self.nixosModules.default
-            ({ ... }: {
-              system.stateVersion = "25.11";
-
-              fonts.packages = [ pkgs.adwaita-fonts pkgs.dejavu_fonts ];
-              environment.systemPackages = [
-                pkgs.fastfetch
-                pkgs.htop
-                virtwl.packages.x86_64-linux.proxy
-                pkgs.wayland-utils
-                pkgs.weston
-                pkgs.waycheck
-                pkgs.vulkan-tools
-                pkgs.glmark2
-                pkgs.mesa-demos
-                pkgs.xorg.xeyes
-                pkgs.xterm
-                # pkgs.vkquake # build broken: Program 'spirv-remap' not found
-                pkgs.veloren
-                pkgs.kdePackages.kate
-                pkgs.adwaita-icon-theme
-                pkgs.amberol
-                pkgs.bustle
-                pkgs.d-spy
-                pkgs.gnome-text-editor
-                pkgs.firefox
-                pkgs.ffmpeg-full
-                pkgs.mpv
-                pkgs.libva-utils
-                pkgs.tailscale
-                pkgs.zerotierone
-                pkgs.localsend
-              ];
-            })
-          ];
-        };
-
       });
 }