volume: use qemuimg and remove custom qemuRebase

This patch substitutes a custom qemuRebase function with the more
standard implementation in qemuimg module.

Change-Id: Icd05300bb31dae68a67ea8c2a3252b28a8d7978c
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26901
Reviewed-by: Dan Kenigsberg <danken@redhat.com>