title: Get ORM config name from EnityManager object in EntityListener
body: <p>Hi i'm working on preupdate function like</p>
  
  <pre><code>public function preUpdate(PreUpdateEventArgs $args){
      $em = $args-&gt;getEntityManager();
  }
  </code></pre>
  
  <blockquote>
    <p>I would like to know if it's possible to get the config name of the entity
    manager with $em object (Doctrine\ORM\EntityManager)</p>
  </blockquote>
  
  <p>In my config.yml</p>
  
  <pre><code>orm:
      ...
      entity_managers:
          connection1:
              ...
          connection2:
              ...
  </code></pre>
  
  <p>I try to get string "connection1" or "connection2"</p>
  
  <p>Thanks :)</p>