image: Calculation of chain to remove is inaccurate

The if condition will always return false,
therefore list 'rmChain' will always be composed of only one volume.

Change-Id: I248c51727584a35d1f439e57c93613caa9f369a9
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19852
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Daniel Erez <derez@redhat.com>