Fix bad  image tagging when postzeroing.

The postzero code fails to remove the image tag from the volume
and adds an extra tag without the right prefix.

Change-Id: Ica92826f2d6937018fc3c384bf1377df7f541f2b
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11593
Reviewed-by: Dan Kenigsberg <danken@redhat.com>