ceph-disk: make list_partition behave with unusual device names

When you get device names like sdaa you do not want to mistakenly conclude that
sdaa is a partition of sda.  Use /sys/block/$device/$partition existence
instead.

Fixes: #5211
Signed-off-by: Alexandre Maragone <alexandre.maragone@inktank.com>
ceph-disk: handle /dev/foo/bar devices throughout

Assume the last component is the unique device name, even if it appears
under a subdir of /dev.

Signed-off-by: Sage Weil <sage@inktank.com>