qom: Assert that public types have a non-NULL parent field

This protects against unwanted effects of changing TYPE_OBJECT from
NULL to a string.

Suggested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
