iothread: add "iothread" qdev property type
Add a "iothread" qdev property type so devices can be hooked up to an
IOThread from the comand-line:
qemu -object iothread,id=iothread0 \
-device some-device,x-iothread=iothread0
Note that Paolo Bonzini <pbonzini@redhat.com> has suggested using QOM
links instead. This way the relationship between the objects is
reflected in QOM. There are currently shortcomings of
object_property_add_link() which prevent this use case. I will attempt
to fix them and move to QOM links in a separate series.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>