pylint: fix format string

Change-Id: I36ce9cdb63c49c0058af1f357badd13b1ae6be2b
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24412
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>