image: use successor volume size when merging

Volumes in the same image chain might have different capacity since the
introduction of the disk resize feature. This means that when we merge
volumes the ancestor should get the new size from the successor in order
to be able to contain the additional data that we are collapsing.

Change-Id: Ic0bfd2ffa080bf89709ae543043b52aad27eb759
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17093
Reviewed-by: Eduardo <ewarszaw@redhat.com>