Improve grub config handling

< Diziet> [...] this xen-hypervisor-*.xen.cfg update-grub config file fragment 
          is a bit of a bodge.
< Diziet> I inherited it from Ubuntu (I took 4.8.0-0ubuntu2, mostly)
< Diziet> The things that are wrong with it is that 1. it won't work for anyone not using grub
          2. it's a pretty ugly way of fixing the problem that otherwise you install Xen and
          reboot and still reboot into baremetal