include/hw/virtio: more comment for VIRTIO_F_BAD_FEATURE

When debugging a new vhost user you may be surprised to see
VHOST_USER_F_PROTOCOL getting squashed in the maze of
backend_features, acked_features and guest_features. Expand the
description here to help the next poor soul trying to work through
this.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20220802095010.3330793-6-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/hw/virtio/virtio.h

@@ -24,7 +24,12 @@
 #include "qom/object.h"
 #include "hw/virtio/vhost.h"
 
-/* A guest should never accept this.  It implies negotiation is broken. */
+/*
+ * A guest should never accept this. It implies negotiation is broken
+ * between the driver frontend and the device. This bit is re-used for
+ * vhost-user to advertise VHOST_USER_F_PROTOCOL_FEATURES between QEMU
+ * and a vhost-user backend.
+ */
 #define VIRTIO_F_BAD_FEATURE		30
 
 #define VIRTIO_LEGACY_FEATURES ((0x1ULL << VIRTIO_F_BAD_FEATURE) | \