kvm tools: Process virtio-blk requests in parallel

kvm tools: Process virtio-blk requests in parallel

Process multiple requests within a virtio-blk device's vring
in parallel.

Doing so may improve performance in cases when a request which can
be completed using data which is present in a cache is queued after
a request with un-cached data.

bonnie++ benchmarks have shown a 6% improvement with reads, and 2%
improvement in writes.

Suggested-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>