Set QEMUVGA back to qxl for x86_64, unset it for aarch64
    
    qemu 2.11.2 fixes #1403343, so we can go back to using qxl for
    x86_64 VMs. Also, setting QEMUVGA doesn't actually do anything
    on aarch64 (os-autoinst ignores it and sets up virtio graphics
    instead), so let's not set it to avoid confusion.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>