Allow max_pages to be set to less than tot_pages

Allow max_pages to be set to less than tot_pages

The memory allocation code sometimes needs to enforce that a guest
that's been told to balloon down isn't going to expand further
(because it's still executing a previous balloon-up operation).  That
means being able to set the desired max_pages even before the balloon
driver has brought tot_pages down to the right level.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>