It cost's me some nerves and time to figure out how to create a vagrant image from a libvirt kvm vm and how to modify an existing one. Thanks to pl_rock from stackexchange for the awesome start.
- First of all you have to install a new vm as usual. I've installed a new vm with Ubuntu 16.04 LTS. I'm not sure if it's really neccessary but set the root password to "vagrant", just to be sure.
- Connect to your VM via ssh or terminal and do the following steps.