virtio-blk: Drop x-data-plane option

The official way of enabling dataplane is through the "iothread"
property that references an iothread object created by "-object
iothread".  Since the old "x-data-plane=on" way now even crashes, it's
probably easier to just drop it:

$ qemu-system-x86_64 -drive file=null-co://,id=d0,if=none \
    -device virtio-blk-pci,drive=d0,x-data-plane=on

ERROR:/home/fam/work/qemu/qom/object.c:1515:
object_get_canonical_path_component: assertion failed: (obj->parent != NULL)
Aborted

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1449485967-19240-1-git-send-email-famz@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
