virtio-blk: replace x-iothread with iothread link property

Up until now -device virtio-blk-pci,x-iothread=<id> was used to assign
an IOThread.  This was a temporary solution while we cleaned up QOM link
properties.

This patch switches over to a QOM link property since it is now possible
to restrict the setter to unrealized instances and automatically unref
the IOThread when the virtio-blk-pci device is freed.

Since the "iothread" property is a QOM property and not a qdev property,
we must alias it explicitly for virtio-blk-pci, as well as CCW and
s390-virtio.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
qdev: drop iothread property type

The iothread property type is no longer used and can be removed.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
virtio-blk: remove need for explicit x-data-plane=on option

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>