DDF: handle fake RAIDs with changing subarray UUIDs

DDF: handle fake RAIDs with changing subarray UUIDs

Some fake RAID BIOSes (in particular, LSI ones) change the
VD GUID at every boot. These GUIDs are not suitable for
identifying an array. Luckily the header GUID appears to
remain constant.

We construct a pseudo-UUID from the header GUID and those
properties of the subarray that we expect to remain constant.
This is only array name and index; all else might change e.g.
during grow.

Don't do this for all non-MD arrays, only for those known
to use varying volume GUIDs.

This patch obsoletes my previous patch "DDF: new algorithm
for subarray UUID"

Signed-off-by: Martin Wilck <mwilck@arcor.de>
Signed-off-by: NeilBrown <neilb@suse.de>