'Return name of base image used for backup.
Incremental backups use a new base name so we support old and new style
format.'

def _get_backup_base_name(self, volume_id, backup_id=None, diff_format=False):

    if diff_format:
        return utils.convert_str(('volume-%s.backup.base' % volume_id))
    else:
        if (backup_id is None):
            msg = _('Backup    id    required')
            raise exception.InvalidParameterValue(msg)
        return utils.convert_str(('volume-%s.backup.%s' % (volume_id, backup_id)))
