gui-linux: Add reproducer for hardstop issue

When calling vm.Stop() while using the GUI, the VM stops, but
the GUI is not closed and the application keeps running.
Using vm.RequestStop() does not have this issue.

This commit modifies example/gui-linux to exhibit the issue.

example/gui-linux/main.go

@@ -77,6 +77,17 @@ func run(ctx context.Context) error {
 		}
 	}()
 
+	go func() {
+		if !vm.CanStop() {
+			log.Println("cannot stop vm forcefully")
+			return
+		}
+		time.Sleep(10 * time.Second)
+		log.Println("calling vm.Stop()")
+
+		vm.Stop()
+	}()
+
 	// cleanup is this function is useful when finished graphic application.
 	cleanup := func() {
 		for i := 1; vm.CanRequestStop(); i++ {