feat(virtualisation): update libvirtd enablement logic

- Disable libvirtd explicitly in m4nix configuration.
- Use `lib.mkDefault` for libvirtd enablement in base services to allow overrides.

systems/aarch64-linux/m4nix/default.nix

@@ -63,6 +63,7 @@ with lib.metacfg;
   virtualisation = {
     docker.enable = true;
     podman.dockerCompat = false;
+    libvirtd.enable = false;
   };
 
   system.autoUpgrade = {