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.