Get list of VTrees from ScaleIO cluster
        :return: List of VTree objects - Can be empty of no VTrees exist
        :rtype: VTree object

def vtrees(self):
        """
        Get list of VTrees from ScaleIO cluster
        :return: List of VTree objects - Can be empty of no VTrees exist
        :rtype: VTree object
        """
        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