Set the subvolume as the current default.

def _set_default(path, dest, name):
    '''
    Set the subvolume as the current default.
    '''
    subvol_id = __salt__['btrfs.subvolume_show'](path)[name]['subvolume id']
    return __salt__['btrfs.subvolume_set_default'](subvol_id, dest)