[IA64] Allow simulator to use bigger disks

The simscsi code at present overflows an int if it's given a large
disk image.  The attached patch increases the possible size to 128G.
While it's unlikely that anyone will want to use SKI with such a
large drive, the same framework is currently being used for various
virtualisation experiments.

Signed-off-by: Peter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>