volume: fix qemuRebase function signature

A sloppy rebase of commit bb50245 hid a necessary update of the
qemuRebase function signature.

Change-Id: I707644ccb9530c83b05a412bb4a74fd07a978e7c
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27527
Reviewed-by: Dan Kenigsberg <danken@redhat.com>