anaconda: regenerate grub.cfg using grub2-mkconfig

Normally anaconda uses grubby to update bootloader configuration. But
unfortunately it breaks grub configuration (removes trailing "=", so
empty variable assignment xen_rm_opts= is broken). Instead of reworking
the whole bootloader support in anaconda, just regenerate grub.cfg at
the end.

anaconda/data/post-scripts/50-qubes.ks

@@ -5,5 +5,6 @@ for initrd in /boot/initramfs-*.img; do
    version=`basename $initrd .img|cut -f 2- -d-`
    dracut --force $initrd $version
 done
+grub2-mkconfig -o /boot/grub2/grub.cfg
 
 %end