virtio-blk: trivial code optimization 1. avoid possible superflous checking 2. make code more robustness ["make code more robustness" refers to avoiding integer underflows/overflows. --Stefan] Signed-off-by: Gonglei <arei.gonglei@huawei.com> Message-id: 1447207166-12612-1-git-send-email-arei.gonglei@huawei.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>