Use qxl for x86_64 UEFI, virtio for ppc64(le)
    
    virtio graphics still seem to suffer from RHBZ#1403365, and also
    os-autoinst believes they don't support snapshotting. So let's
    try qxl for x86_64 UEFI. *That* may still suffer from #1403343,
    but oh well, seems like we have no good choices here.
    
    It looks like ppc64 also suffers from the Plymouth bug that's
    affecting x86_64 UEFI + 'std' graphics, so let's use virtio
    there - qxl apparently isn't available on ppc64 VMs, at least it
    doesn't work in our deployment.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>