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
Backport: cuttlefish
Signed-off-by: Alexandre Maragone <alexandre.maragone@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>