Looks like adding RAID 5/6 support may be easier than most
could immagine. The code, as written appears to be safe and
logical creating a RAID 5 or RAID 6 volume.
Not that we can really test this in CI, but it seems only
validation code needs to be changed to loosen the constraint.
Change-Id: Ib891b3c97f0bfb02af3b59581a451c4b25e03b85