index can be reused by other virtio device.

Cc: stable@vger.kernel.org
Signed-off-by: Nweiping zhang <zhangweiping@didichuxing.com>
Reviewed-by: NCornelia Huck <cohuck@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>