volume: unify the backing file path computation

This patch unifies the backing file path computation logic in a single
function (getBackingVolumePath).

Change-Id: I7575f4dd3c6918dfee72ac4c23f27a56a8390b9b
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26918
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>