include/hw: temporarily disable deletion of versioned machine types

include/hw: temporarily disable deletion of versioned machine types

The new deprecation and deletion policy for versioned machine types is
being introduced in QEMU 9.1.0.

Under the new policy a number of old machine types (any prior to 2.12)
would be liable for immediate deletion which would be a violation of our
historical deprecation and removal policy

Thus automatic deletions (by skipping QOM registration) are temporarily
gated on existance of the env variable "QEMU_DELETE_MACHINES" / QEMU
version number >= 10.1.0. This allows opt-in testing of the automatic
deletion logic, while activating it fully in QEMU >= 10.1.0.

This whole commit should be reverted in the 10.1.0 dev cycle or shortly
thereafter.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240620165742.1711389-10-berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>