vhost_rs: Add INFLIGHT_SHMFD protocol feature

Add to the vhost_rs crate the INFLIGHT_SHMFD protocol feature that was
missing from the list. The feature has been recently introduced in
vhost-user specification, which is the reason why it was missing.

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

vhost_rs/src/vhost_user/message.rs

@@ -346,6 +346,8 @@ bitflags! {
         const SLAVE_SEND_FD = 0x0000_0400;
         /// Allow the slave to register a host notifier.
         const HOST_NOTIFIER = 0x0000_0800;
+        /// Support inflight shmfd.
+        const INFLIGHT_SHMFD = 0x0000_1000;
     }
 }