tools: disable building qemu-trad per default

Using qemu-traditional as device model is deprecated for some time now.

So change the default for building it to "disable". This will affect
ioemu-stubdom, too, as there is a direct dependency between the two.

Today it is possible to use a PVH/HVM Linux-based stubdom as device
model. Additionally using ioemu-stubdom isn't really helping for
security, as it requires to run a very old and potentially buggy qemu
version in a PV domain. This is adding probably more security problems
than it is removing by using a stubdom.

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

@@ -18,6 +18,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    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.
+ - qemu-traditional based device models (both, qemu-traditional and ioemu-stubdom) will
+   no longer be built per default. In order to be able to use those, configure needs to
+   be called with "--enable-qemu-traditional" as parameter.
 
 ## [4.15.0 UNRELEASED](https://xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.15.0) - TBD