All these errors are caused by a buggy guest: QEMU should not exit.

With this patch, if virtio_blk_handle_request() detects a buggy request, it
marks the device as broken and returns an error to the caller so it takes
appropriate action.

In the case of virtio_blk_handle_vq(), we detach the request from the
virtqueue, free its allocated memory and stop popping new requests.
We don't need to bother about multireq since virtio_blk_handle_request()
errors out early and mrb.num_reqs == 0.

In the case of virtio_blk_dma_restart_bh(), we need to detach and free all
queued requests as well.
Signed-off-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>