include/hw: document vhost_dev feature life-cycle

Try and explicitly document the various state of feature bits as
related to the vhost_dev structure. Importantly the backend_features
can advertise things like VHOST_USER_F_PROTOCOL_FEATURES which is
never exposed to the driver and is only present in the vhost-user
feature negotiation.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220802095010.3330793-7-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

include/hw/virtio/vhost.h

@@ -86,8 +86,11 @@ struct vhost_dev {
     /* if non-zero, minimum required value for max_queues */
     int num_queues;
     uint64_t features;
+    /** @acked_features: final set of negotiated features */
     uint64_t acked_features;
+    /** @backend_features: backend specific feature bits */
     uint64_t backend_features;
+    /** @protocol_features: final negotiated protocol features */
     uint64_t protocol_features;
     uint64_t max_queues;
     uint64_t backend_cap;