xen/x86: Increase xen_e820_map to E820_X_MAX possible entries

On systems with sufficiently large e820 tables, and several IOAPICs, it
is possible for the XENMEM_machine_memory_map callback (and its
counterpart, XENMEM_memory_map) to attempt to return an e820 table with
more than 128 entries.  This callback adds entries to the BIOS-provided
e820 table to account for IOAPIC registers, which, on sufficiently large
systems, can result in an e820 table that is too large to copy back into
xen_e820_map.

This change simply increases the size of xen_e820_map to E820_X_MAX to
ensure that there is enough room to store the entire e820 map returned
from this callback.

Signed-off-by: Alex Thorlton <athorlton@sgi.com>
Suggested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>