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.
Signed-off-by: Javier Martinez Canillas <firstname.lastname@example.org>
Tested-by: Steven Haigh <email@example.com>