virtio_blk: enable VQs early

virtio spec requires drivers to set DRIVER_OK before using VQs.
This is set automatically after probe returns, virtio block violated this
rule by calling add_disk, which causes the VQ to be used directly within
probe.

To fix, call virtio_device_ready before using VQs.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>