OK, I've solved this problem. I seems that the old CENTOS7_64.tar.xz basefile in http://fai-project.org/download/basefiles/ caused all the problems. I've created a new one, which now works.
Using this patch, you can now add the FAI class LVM to your CentOS install client and use a LVM partitioning with CentOS 7. https://github.com/faiproject/fai/commit/236d8e9c4762ea235af757968607e0cde33708f1 -- regards Thomas
