vhost: correct misleading printing information

Guest moved avail idx not used idx when we need to print log if
'(vq->avail_idx - last_avail_idx) > vq->num', so fix it.

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Message-Id: <20240311082109.46773-1-xianting.tian@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>