Fix order-of-evaluation issue in xc_domain_dumpcore. The intent is

Fix order-of-evaluation issue in xc_domain_dumpcore.  The intent is
to move blocks of memory 4096 pages at a time, and a buffer is
allocated for this.  Unfortunately, the #define is without
parentheses and %/* are the same order and evaluated left to right.
Result: very big buffer being used 4 pages at a time.

Signed-off-by: Ben Thomas (bthomas@virtualiron.com)