[IA64] Increase hypervisor memory reservation

[IA64] Increase hypervisor memory reservation

Recent additions to the xen codebase have inflated the memory requirements
of the hypervisor a bit. To compensate, we need to increase the amount of
memory we try to reserve for the hypervisor when allocating "all" system
memory to dom0.

Fixes kernel panics on a 2GB rx2600 in our lab, as well as on misc. test
systems with between 16 and 128GB when they're configured to allocate "all"
memory to dom0.

Signed-off-by: Jarod Wilson <jwilson@redhat.com>