title: qtmqtt can not connect to server
body: <p>QtMqtt cannot connect to the server, but I can connect normally using other test software.The server is mosquitto on Ubuntu.</p>
  
  <pre><code> m_client= new QMqttClient(this);
   m_client-&gt;setProtocolVersion(QMqttClient::MQTT_3_1_1);
   m_client-&gt;setPort(1883);
   m_client-&gt;setHostname("127.0.0.1");
   m_client-&gt;setClientId("qt");
   m_client-&gt;connectToHost();
  
  connect(m_client,SIGNAL(stateChanged(ClientState)),this,SLOT(slot_stateChanged()),Qt::UniqueConnection);
  
  void slot_stateChanged()
  {
      qDebug() &lt;&lt; "mqtt stsate" &lt;&lt; _client-&gt;state();
  }
  </code></pre>