contrib/vhost-user-blk: introduce a vhost-user-blk sample application
This commit introduces a vhost-user-blk backend device, it uses UNIX
domain socket to communicate with QEMU. The vhost-user-blk sample
application should be used with QEMU vhost-user-blk-pci device.
To use it, complie with:
make vhost-user-blk
and start like this:
vhost-user-blk -b /dev/sdb -s /path/vhost.socket