nixos/libvirtd: utilize onShutdown option (#49480)

`services.virtualisation.libvirtd.onShutdown` was previously unused.
While suspending a domain on host shutdown is the default, this commit
makes it so domains can be shut down, also.

nixos/modules/virtualisation/libvirtd.nix

@@ -196,6 +196,8 @@ in {
       wantedBy = [ "multi-user.target" ];
       path = with pkgs; [ coreutils libvirt gawk ];
       restartIfChanged = false;
+
+      environment.ON_SHUTDOWN = "${cfg.onShutdown}";
     };
 
     systemd.sockets.virtlogd = {