qom-test: Test qom-list on link<> properties

But don't test their properties, otherwise we will recurse forever.
Their properties are already tested when we encounter them as child<>
properties elsewhere in the hierarchy, like /machine/unattached/...

This would have caught the crash fixed by 92b3eead.

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>