The x-data-plane=on|off option is no longer useful because the
iothread=<iothread> option conveys the same information plus which
IOThread to use.
Do not delete x-data-plane=on|off yet as a convenience to people using
this legacy experimental option. We will drop it in QEMU 2.2.
Instead, turn on data-plane when either x-data-plane=on or
iothread=<iothread> are used. The following command-line uses
data-plane:
qemu -device virtio-blk-pci,iothread=foo,drive=drive0
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>