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>
