Handle xen module loading (somewhat) better
    
    You'll still need to actually install grub2-${efiarch}-modules and then
    use grub2-install to install the xen modules in /boot/grub2/,
    but this should handle actually loading them from the grub config file.
    
    Resolves: rhbz#1486002
    
    Signed-off-by: Peter Jones <pjones@redhat.com>