vhost_user_backend: move protocol_features to the backend

Extend VhostUserBackend trait with protocol_features(), so device
backend implementations can freely define which protocol features they
want to support.

Signed-off-by: Sergio Lopez <slp@redhat.com>

src/bin/vhost_user_net.rs

@@ -433,6 +433,10 @@ impl VhostUserBackend for VhostUserNetBackend {
             | VhostUserVirtioFeatures::PROTOCOL_FEATURES.bits()
     }
 
+    fn protocol_features(&self) -> VhostUserProtocolFeatures {
+        VhostUserProtocolFeatures::all()
+    }
+
     fn update_memory(&mut self, mem: GuestMemoryMmap) -> VhostUserBackendResult<()> {
         self.mem = Some(mem);
         Ok(())