xpenology: XEN changed block device prefixes.

The XEN block device driver renames the disks so they prefix
with xvd. Devices with xvd names aren't seen by xpenology.
Here we amend it so that unless a xvd device has been specified
in the XEN config, sd will be used instead, thus enabling
compatibility.
Merge branch 'master' of https://github.com/HuxyUK/xpenology-3.x

Conflicts:
	drivers/block/xen-blkfront.c