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

Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

.gitignore

@@ -54,6 +54,7 @@
 /module_block.h
 /scsi/qemu-pr-helper
 /vhost-user-scsi
+/vhost-user-blk
 /fsdev/virtfs-proxy-helper
 *.tmp
 *.[1-9]