tests: Add s390x-ccw-graphics test case
    
    We have tests for simple guests with graphics for basically
    all other architectures, so it makes sense to include s390x
    too.
    
    The input file was generated by running
    
      $ virt-install \
        --name guest --os-variant fedora29 \
        --vcpus 4 --memory 4096 --disk size=5 \
        --graphics vnc \
        --print-xml
    
    followed by minor tweaks, using a version of virt-manager
    that includes commit 7b9de27a990f.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Acked-by: Michal Privoznik <mprivozn@redhat.com>