grub.d/xen.cfg: fix default entry when using l10n

grub.d/xen.cfg: fix default entry when using l10n

When a user uses a locale that results in translating menu item titles
into another language than English, the hardcoded "Debian GNU/Linux,
with Xen hypervisor" would not match anything.

So, use gettext to make it match the right translated entry.
Also see
- https://bugs.launchpad.net/ubuntu/+source/xen/+bug/1321144
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=865086

Note that (thanks Ian for the info):
* When GRUB_TERMINAL is not empty and set to anything other than
  `gfxterm', grub will not do translation at all, because grub-mkconfig
  thinks that other GRUB_TERMINAL values including `serial' preclude
  non-ASCII characters, and that causes it to set LANG=C. (I have
  GRUB_TERMINAL="serial console", which caused much confusion when
  trying to test all of this).
* Just trying the printf "$(gettext... below is not enough to test if a
  translation shows up. It needs -d grub additionally for gettext, or
  TEXTDOMAIN=grub in the environment, which is probably present when
  this file gets run by update-grub.

Signed-off-by: Hans van Kranenburg <hans@knorrie.org>