99-grub-mkconfig: Disable BLS usage for Xen DomU guests
    
    GRUB is used to boot Xen Dom0 hosts so BLS is supported there, but PV and
    PVH Xen DomU guests are booted with pygrub that doesn't have BLS support.
    
    This means that only HVM Xen DomU guests can support BLS and others need
    a traditional GRUB configuration file that contains menuentry commands.
    
    So to make the GRUB config compatible with all Xen DomU guests, don't use
    the BLS snippets and instead call grub2-mkconfig to regenerate the config
    on each kernel install and removal.
    
    Even when HVM Xen DomU guests can support BLS, just disable for all DomU
    so the configuration is consistent across all Xen DomU guests variants.
    
    And also because even when an installation is made with HVM, this can be
    later changed to a DomU variant that uses pygrub and doesn't support BLS.
    
    Resolves: rhbz#1703700
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
    Tested-by: Steven Haigh <netwiz@crc.id.au>