chore(nix): improve libvirt and virtualisation settings

- Added `qemuOvmf = true` to enable UEFI support in libvirt.
- Restructured `libvirtd` configuration for better readability.

modules/nixos/services/base/default.nix

@@ -1,7 +1,8 @@
-{ config
-, lib
-, pkgs
-, ...
+{
+  config,
+  lib,
+  pkgs,
+  ...
 }:
 with lib;
 with lib.metacfg;
@@ -185,7 +186,10 @@ in
       };
     };
 
-    virtualisation.libvirtd.enable = lib.mkDefault true;
+    virtualisation.libvirtd = {
+      enable = lib.mkDefault true;
+      qemuOvmf = true;
+    };
 
     boot.tmp.useTmpfs = lib.mkDefault true;