KeyValueStore: Fix upgrade from non-exist superblock OSD version error

If user upgrade OSD from Firefly or lower than 0.84, it won't get valid
keyvaluestore backend.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #2429 from yuyuyu101/kvstore-fix-superblock

KeyValueStore: Fix upgrade from non-exist superblock OSD version error

Reviewed-by: Sage Weil <sage@redhat.com>