xen: prevent crashes with non-HIGHMEM 32-bit kernels with largeish memory

If this is a non-HIGHMEM 32-bit kernel, then the page structures only go
up to the limit of addressable memory, even if more memory is physically
present.  Don't try to add that extra memory to the balloon.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>