pc: memhotplug: keep reserved-memory-end broken on 2.4 and earlier machines

it will prevent guests on old machines from seeing
inconsistent memory mapping in firmware/ACPI views.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
