storage: fix pep issue with image.py

This patch fix the if indentation that pep complains.
Introduced by patch 7b4bd1b4.

Change-Id: I82b7e096292799b5c7e7f7f595e094643014ea69
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27273
Reviewed-by: Tal Nisan <tnisan@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>