ceph-disk: add get_partition_base() helper

Return the base devices/disk for a partition device.

Signed-off-by: Sage Weil <sage@redhat.com>
ceph-disk: only fall back to sgdisk for 'list' if blkid seems old

If the blkid doesn't show us any ID_PART_ENTRY_* fields but we know it is
a GPT partition, *then* fallback.  Otherwise, don't bother.

Signed-off-by: Sage Weil <sage@redhat.com>
ceph-disk: warn about falling back to sgdisk (once)

This way the user knows something funny might be up if dmcrypt is in use.

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