hw/virtio: log potentially buggy guest drivers

If the guest driver attempts to use the UNUSED(30) bit it is
potentially buggy as 6.3 Legacy Interface: Reserved Feature Bits
states it "SHOULD NOT be negotiated". For now just log this guest
error.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220802095010.3330793-9-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -2980,6 +2980,13 @@ int virtio_set_features(VirtIODevice *vdev, uint64_t val)
     if (vdev->status & VIRTIO_CONFIG_S_FEATURES_OK) {
         return -EINVAL;
     }
+
+    if (val & (1ull << VIRTIO_F_BAD_FEATURE)) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: guest driver for %s has enabled UNUSED(30) feature bit!\n",
+                      __func__, vdev->name);
+    }
+
     ret = virtio_set_features_nocheck(vdev, val);
     if (virtio_vdev_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX)) {
         /* VIRTIO_RING_F_EVENT_IDX changes the size of the caches.  */