Our current public KVM/VMware/VirtualBox images are not ready to be used that way as they lack support for cloud-init or similar mechanism. You might be able to use guestfish to set a password for user root before starting the image; we do something similar internally.
As an alternative you should be able to use virt-install to create your own images.
Since I am still pretty new to all this I made some rookie mistakes here. What I haven’t known when I originally wrote that post: As soon as libvirt and those dependencies are installed on a hypervisor, one can connect to it via virt-manager. The final bit which was bugging was some sort of ssh trouble. Trying to get a Display of a VM from virt-manager didn’t work and I was asked for my password on the hypervisor over and over again. Using virt-viewer from the command line just worked.