'Manage an existing Hitachi storage volume.
existing_ref is a dictionary of the form:
For HUS 100 Family:
.. code-block:: default
\'ldev\': <logical device number on storage>,
\'unit_name\': <storage device name>
For VSP G1000/VSP/HUS VM:
.. code-block:: default
\'ldev\': <logical device number on storage>,
\'serial_number\': <product number of storage system>'

def manage_existing(self, volume, existing_ref):

    ldev = self._string2int(existing_ref.get('ldev'))
    LOG.info(basic_lib.set_msg(4, volume_id=volume['id'], ldev=ldev))
    return {'provider_location': ldev}
