'Return the volume name of an existing ref.'

def _get_existing_volume_ref_name(self, existing_ref):

    if ('source-name' in existing_ref):
        vol_name = existing_ref['source-name']
    else:
        reason = _('Reference    must    contain    source-name.')
        raise exception.ManageExistingInvalidReference(existing_ref=existing_ref, reason=reason)
    return vol_name
