ui/gtk: Fix focus loss on re-attachment with single VC

When QEMU is launched with a single virtual console (e.g., using
--nodefaults), detaching and then closing the detached window leaves
the main window's notebook without an active focus target which makes
keyboard unfunctional on re-attached VC.

Fix this by explicitly calling gtk_widget_grab_focus on the active
VC's focus widget during the window close handler.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Message-ID: <20260520002645.1910740-1-dongwon.kim@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

ui/gtk.c

@@ -1483,6 +1483,11 @@ static gboolean gd_tab_window_close(GtkWidget *widget, GdkEvent *event,
         vc->gfx.ectx = NULL;
     }
 #endif
+
+    if (vc == gd_vc_find_by_menu(s)) {
+        gtk_widget_grab_focus(vc->focus);
+    }
+
     return TRUE;
 }