blockVolume: prepare volumes for shrinkToOptimalSize

The qemu-img check command needs the entire volume chain in order to
be able to open the image.
Up until now shrinkToOptimalSize worked only for base volumes, in the
other cases it was failing with:

 QImgError: ecode=1, stdout=[], stderr=[
   "Could not open '(image path)': No such file or directory"]

Change-Id: Ic9fceee28932da6895829c7e7efadb735c1ec060
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28009
Reviewed-by: Kevin Wolf <kwolf%redhat.com@gtempaccount.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>