Move the calculation of the Real Mode Area (RMA) size into a helper
function. While we're there clean it up and correct it in a few ways:
* Add comments making it clearer where the various constraints come from
* Remove a pointless check that the RMA fits within Node 0 (we've just
clamped it so that it does)
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>