The images don't have a root password set. You can use libguestfs-tools to set one ...
virt-customize -a <image> --root-password "password:<secret>"
... so you can login after boot
The fedora and centos8 images will boot in both uefi and bios mode. Starting with systemd-nspawn --image=<image> works too, but the images must be converted to raw format first; qemu-img convert ... can be used for that.