99-grub-mkconfig: Fix script condition to exit and remove ppc64 BE check
    
    The ppc64 big endian support ended in F29 and only ppc64le is supported so
    there's no need to check for this architecture.
    
    Also the script wasn't properly tested and the check for Xen DomU machines
    isn't correct. Fix the test condition to exit the script.
    
    Related: rhbz#1703700
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>