code: def vtrees(self):
          
          self.connection._check_login()
          response = self.connection._do_get("{}/{}".format(self.connection._api_url, "types/VTree/instances")).json()
          all_vtrees = []
          for vtree in response:
              all_vtrees.append(
                  SIO_Vtree.from_dict(vtree)
              )
          return all_vtrees
docs: Get list of VTrees from ScaleIO cluster
          :return: List of VTree objects - Can be empty of no VTrees exist
          :rtype: VTree object