EntityFieldQuery get webform with block

  $query = new EntityFieldQuery();
  $query->entityCondition('entity_type', 'node')
          ->entityCondition('bundle', 'webform');
  $result = $query->execute();
  $options = array(
      0 => 'None'
  );
  if (!empty($result['node'])) {
      $nids = array_keys($result['node']);
      $nodes = node_load_multiple($nids);
      foreach ($nodes as $nid => $node) {
        $options[$nid] = $node->label();
      }
  }  
  return $options;

I use the above code to retrieve nodes that are webforms.
What I would like to know is what condition to use to get only the webforms that have the block functionality?