sd: fix volume path returned by linkBCImage

In a previous commit (c072945 One shot prepare) we involuntarily
changed the path used for virtual machine images from:

 /rhev/data-center/<spUUID>/<sdUUID>/images/<imgUUID>/<volUUID>

to:

 /rhev/data-center/mnt/blockSD/<sdUUID>/images/<imgUUID>/<volUUID>

This patch reverts to the previous path in order to minimize the
differences between different versions of vdsm that could lead to
live migration issues, e.g.

 libvirtError: invalid argument: invalid path ... not assigned to
 domain

Change-Id: Iea497b574d9a9a7e0e9b8e234e2bd9b9d983a60c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1059482
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24315
Reviewed-by: Dan Kenigsberg <danken@redhat.com>