ceph-disk: get Nonetype when ceph-disk list with --format plain on single device.

ceph-disk: get Nonetype when ceph-disk list with --format plain on single device.

    with dmcrypt device, ceph-disk list will get Nonetype with the real device
    i.e. /dev/sdb1 for /dev/dm-1, `ceph-disk --list /dev/sdb1` will get Nonetype

    that because the holder we query is `dm-x`, the devices includ the path
    is `sdX`, it cannot match.

    if we list format plain and meet dmcrypt device, query again with the
    holder (dm-x).

    Also change the list_devices() parameter to let other function to use it
    easily.

Signed-off-by: Vicente Cheng <freeze.bilsted@gmail.com>