virtio spec requires drivers to set DRIVER_OK before using VQs.
This is set automatically after restore returns, virtio block violated
this rule on restore by restarting queues, which might in theory
cause the VQ to be used directly within restore.

To fix, call virtio_device_ready before using starting queues.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>