This mode has been introduced in libvirt 7.1.0 (March 2021) and
can be already used today with
--cpu mode=maximum
This is however slightly inconvenient to type and is not
consistent with the special treatment that the other modes
(host-passthrough, host-model) get.
Introduce a proper special mode for it.
Signed-off-by: Andrea Bolognani <abologna@redhat.com>