vhost-user-blk: Add MQ support in backend

Adding the num_queues parameter for vhost-user-blk backend, which
can enable MQ support in the backend.

This patch has enabled the MQ support from handle_event, and the
vhost-user-backend crate will enable multiple threads to call this
handle_event to handle read/write operations.

Signed-off-by: Yang Zhong <yang.zhong@intel.com>

src/bin/vhost_user_blk.rs

@@ -25,8 +25,8 @@ fn main() {
                 .long("block-backend")
                 .help(
                     "vhost-user-block backend parameters \"image=<image_path>,\
-                     sock=<socket_path>,readonly=true|false,\
-                     direct=true|false\"",
+                     sock=<socket_path>,num_queues=<number_of_queues>,\
+                     readonly=true|false,direct=true|false\"",
                 )
                 .takes_value(true)
                 .min_values(1),