During image-based provisioning into Libvirt compute-resource I found that newly created VM has "network" as first boot device, which is incorrect.

The possible suspect is missing "image_id" that is supposed to be passed and considered here:

https://github.com/theforeman/foreman/blob/develop/app/models/compute_resources/foreman/model/libvirt.rb#L112

Also, the QCOW image created during the same VM create operation does not have a backing image specified at all making such VM unbootable off that disk/image. Again, missing "image_id" may be at play, IMHO.