ceph-volume: fix inventory without /dev/vg/lv (slashed paths)

ceph-volume: fix inventory without /dev/vg/lv (slashed paths)

Ths makes ceph-volume use UdevData.preferred_block_path() in
get_devices() so it keeps /dev/vg/lv (slashed path form) when
it exists, else /dev/mapper/<name> (dashed path form).

This is needed for thin-pool LVs and environments where udev
does not create slashed paths.

Fixes: https://tracker.ceph.com/issues/75615
Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>
(cherry picked from commit 0e96c625f69179126f3472761d276f2c36aa7b29)
(cherry picked from commit 567590f7fff95a06fb29fec91a167be54febbd1c)