image: produce volume in order to shrink after merge

Create of new volume in case of raw base volume,
causes us to work on a stale object when shrinking
the merged volume.

Change-Id: I5b0969c3144b51ff22a7f5eb756563cf178ffb36
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1015071
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19883
Reviewed-by: Ayal Baron <abaron@redhat.com>