code: func (pm *VolumePluginMgr) FindPersistentPluginBySpec(spec *Spec) (PersistentVolumePlugin, error) {
  	volumePlugin, err := pm.FindPluginBySpec(spec)
  	if err != nil {
  		return nil, fmt.Errorf("Could not find volume plugin for spec: %#v", spec)
  	}
  	if persistentVolumePlugin, ok := volumePlugin.(PersistentVolumePlugin); ok {
  		return persistentVolumePlugin, nil
  	}
  	return nil, fmt.Errorf("no persistent volume plugin matched")
  }
docs: // FindPersistentPluginBySpec looks for a persistent volume plugin that can
  // support a given volume specification.  If no plugin is found, return an
  // error