Look, it seems you don't understand my original question: there's clearly an API change between 1.5.1 and 5.1.2, right? No big deal, since it was announced beforehand. However I cannot find anything in the changelogs that would mention the changes to the <ovf:VirtualSystem> element.
Have a look at the two XML files I attached: there's a vcloud:RuntimeInfoSection element in the first file, however it's missing in the second file.
The first one, ovf-1.5.1.xml, was fetched using https://vclouddirector-1-5-1.local/api/vAppTemplate/vappTemplate-bdd0b603-0460-4f7f-8ce1-d21c9186aa96/ovf (vclouddirector-1-5-1.local is the public API URL for our "old" vCloudDirector, version 1.5.1 build 622844).
The second one, ovf-5.1.2.xml, was fetched using https://vclouddirector-5-1-2.local/api/vAppTemplate/vappTemplate-bdbe87ec-c76f-4f89-81ab-ccd651d0a28b/ovf (vclouddirector-5-1-2.local is the public API URL for our "new" vCloudDirector, version 5.1.2 build 1068441).
All VMs in those vAppTemplates had VMware Tools installed.
I hope this makes my question a bit clearer.
Regards,
Igor