virtio/ringtest: virtio_ring: fix up need_event math

last kicked event index must be updated unconditionally:
even if we don't need to kick, we do not want to re-check
the same entry for events.

Reported-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>