virtio-devices, vmm: Add rate limiter for the TX queue of virtio-net

Partially fixes: #1286

Signed-off-by: Bo Chen <chen.bo@intel.com>

vhost_user_net/src/lib.rs

@@ -111,6 +111,7 @@ impl VhostUserNetThread {
                 epoll_fd: None,
                 counters: NetCounters::default(),
                 tap_event_id: 2,
+                tx_rate_limiter: None,
             },
         })
     }