pc: Leave max apic_id_limit only in legacy cpu hotplug code

That's enough to make old code that depends on it
to prevent QEMU starting with more than 255 CPUs.

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