virtio-devices: vhost_user: Set features once acknowledged by guest

Make sure the virtio features are set upon device activation. At the
time the device is activated, we know the guest acknowledged the
features, which mean it's safe to set them back to the backend.

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

virtio-devices/src/vhost_user/vu_common_ctrl.rs

@@ -74,7 +74,11 @@ pub fn setup_vhost_user(
     queues: Vec<Queue>,
     queue_evts: Vec<EventFd>,
     virtio_interrupt: &Arc<dyn VirtioInterrupt>,
+    acked_features: u64,
 ) -> Result<()> {
+    vu.set_features(acked_features)
+        .map_err(Error::VhostUserSetFeatures)?;
+
     // Let's first provide the memory table to the backend.
     update_mem_table(vu, mem)?;