vhost_user_backend: Provide the thread ID to handle_event()

By adding a "thread_id" parameter to handle_event(), the backend crate
can now indicate to the backend implementation which thread triggered
the processing of some events.

This is applied to vhost-user-net backend and allows for simplifying a
lot the code since each thread is identical.

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

src/bin/vhost_user_fs.rs

@@ -221,6 +221,7 @@ impl<F: FileSystem + Send + Sync + 'static> VhostUserBackend for VhostUserFsBack
         device_event: u16,
         evset: epoll::Events,
         vrings: &[Arc<RwLock<Vring>>],
+        _thread_id: usize,
     ) -> VhostUserBackendResult<bool> {
         if evset != epoll::Events::EPOLLIN {
             return Err(Error::HandleEventNotEpollIn.into());