vm iface stats: fix arithmetic

My commit 672ff43 has a serious arithmetic flaw: the modulu must be
taken on the sample difference!

Bug-Url: https://bugzilla.redhat.com/1062166
Change-Id: Ie356d5f96c73b65925b38f6149d5eb36f43023ad
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25960
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Martin Pavlik <xpavli44@gmail.com>