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The stubdom based pv-grub is using a very outdated version of grub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>

CHANGELOG.md

@@ -16,6 +16,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    appearing in 4.12.2 and 4.11.4). Prior (4.13...4.15-like) behavior can be arranged for
    either by enabling the IOMMU_QUARANTINE_SCRATCH_PAGE setting at build (configuration) time
    or by passing "iommu=quarantine=scratch-page" on the hypervisor command line.
+ - pv-grub stubdoms will no longer be built per default. In order to be able to use pv-grub
+   configure needs to be called with "--enable-pv-grub" as parameter.
 
 ## [4.15.0 UNRELEASED](https://xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.15.0) - TBD