vhost_user_backend: Add new crate

The purpose of this new crate is to provide a common library to all
vhost-user backend implementations. The more is handled by this library,
the less duplication will need to happen in each vhost-user daemon.

This crate relies a lot on vhost_rs, vm-memory and vm-virtio crates.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/queue.rs

@@ -232,8 +232,8 @@ pub struct Queue {
     /// Guest physical address of the used ring
     pub used_ring: GuestAddress,
 
-    next_avail: Wrapping<u16>,
-    next_used: Wrapping<u16>,
+    pub next_avail: Wrapping<u16>,
+    pub next_used: Wrapping<u16>,
 }
 
 impl Queue {