ceph-disk: make is_held() smarter about full disks

Handle the case where the device is a full disk.  Make the partition
check a bit more robust (don't make assumptions about naming aside from
the device being a prefix of the partition).

Signed-off-by: Sage Weil <sage@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>