Move the x-data-plane property. Originally it was outside since not
every transport may wish to support dataplane. But that makes little
sense when we have a dedicated CONFIG_VIRTIO_BLK_DATA_PLANE ifdef
already.
This move makes it easier to switch to property aliases in the next
patch.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
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