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>
