tests/test-qmp-event: fix for GLib < 2.31

On old GLib, the test needs a g_thread_init call.
Reported-by: NWenchao Xia <wenchaoqemu@gmail.com>
Tested-by: NWenchao Xia <wenchaoqemu@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

tests/test-qmp-event.c

@@ -251,6 +251,12 @@ static void test_event_d(TestEventData *data,
 
 int main(int argc, char **argv)
 {
+#if !GLIB_CHECK_VERSION(2, 31, 0)
+    if (!g_thread_supported()) {
+       g_thread_init(NULL);
+    }
+#endif
+
     qmp_event_set_func_emit(event_test_emit);
 
     g_test_init(&argc, &argv, NULL);