stubdom: disable building pv-grub

stubdom: disable building pv-grub

The stubdom based pv-grub is using a very outdated version of grub
(0.97) and should not be used any longer. Mainline grub has support for
PV guests for a long time now, so that should be used as a boot loader
of a PV domain.

So disable building pv-grub per default. In case someone really wants
to continue using it he/she can still use a pv-grub binary from an older
Xen version or manually enable building it via:

  configure --enable-pv-grub

[ This was already disabled in osstest by 8dee6e333622
  "make-flight: Drop pvgrub (pvgrub1) tests" -iwj ]

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>