tests/qtest: switch to using event callbacks for STOP event

Change the migration test to use the new qtest event callback to watch
for the stop event. This ensures that we only watch for the STOP event
on the source QEMU. The previous code would set the single 'got_stop'
flag when either source or dest QEMU got the STOP event.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230601161347.1803440-6-berrange@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

tests/qtest/migration-test.c

@@ -43,6 +43,7 @@
 unsigned start_address;
 unsigned end_address;
 static bool uffd_feature_thread_id;
+static bool got_stop;
 
 /*
  * Dirtylimit stop working if dirty page rate error
@@ -703,6 +704,9 @@ static int test_migrate_start(QTestState **from, QTestState **to,
                                  ignore_stderr);
     if (!args->only_target) {
         *from = qtest_init(cmd_source);
+        qtest_qmp_set_event_callback(*from,
+                                     migrate_watch_for_stop,
+                                     &got_stop);
     }
 
     cmd_target = g_strdup_printf("-accel kvm%s -accel tcg%s%s "