Improve --libvirt default experience
    
    Set `force_imageurl` to `True` by default. And added instructions
    on using disposable clients (via `--libvirt`) to the readme.
    
    File-name convetions for base images in `imagesdir` are described
    both in config, and code.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D775