common/blkdev: handle devices with ID_MODEL as "LVM PV ..." but valid ID_MODEL_ENC

common/blkdev: handle devices with ID_MODEL as "LVM PV ..." but valid ID_MODEL_ENC

Some devices appear like so:

E: ID_MODEL=LVM PV LUClYG-Oyte-jcM6-npfZ-ncsl-ycL0-bkOH0m on /dev/sdn
E: ID_MODEL_ENC=WDC\x20WDS200T2B0A-00SM50\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20
E: ID_SERIAL=WDC_WDS200T2B0A-00SM50_183503800168
E: ID_SERIAL_SHORT=183503800168

To avoid upsetting our prioritization, defer to ID_MODEL_ENC over ID_MODEL
if ID_MODEL has "LVM PV" in it (as opposed to just using ID_SERIAL).

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