'Returns the volume name of an existing reference.
Checks if an existing volume reference has a source-name element.
If source-name is not present an error will be thrown.'

def _get_existing_volume_ref_name(self, existing_ref):

    if ('source-name' not in existing_ref):
        reason = _('Reference    must    contain    source-name.')
        raise exception.ManageExistingInvalidReference(existing_ref=existing_ref, reason=reason)
    return existing_ref['source-name']
