qom: Add a test case for complex property finalization

Devices have some quite complex object child/link relationships
which place some requirements on the object_property_del_all()
function to consider that properties can be modified while
being iterated over.

This extends the QOM property test case to replicate the
device like structure and expose any potential bugs in the
object_property_del_all() function.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
