nvme: also provide a UUID in the WWID sysfs attribute

The WWID sysfs attribute can provide multiple means of a World Wide ID
for a NVMe device. It can either be a NGUID, a EUI-64 or a concatenation
of VID, Serial Number, Model and the Namespace ID in this order of
preference.

If the target also sends us a UUID use the UUID for identification and
give it the highest priority.

This eases generation of /dev/disk/by-* symlinks.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>