99-grub-mkconfig: Disable BLS usage for Xen machines
    
    PV and PVH Xen DomU guests boot with pygrub that doesn't have BLS support.
    Also Xen Dom0 use the menuentries from 20_linux_xen and not the ones from
    10_linux. So BLS support needs to be disabled for both Xen Dom0 and DomU
    and use a traditional grub.cfg file generated by the grub2-mkconfig tool.
    
    Resolves: rhbz#1703700
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
    Tested-by: Steven Haigh <netwiz@crc.id.au>