util: Avoid shadow of 'ulong' in virMemoryMaxValue
Old compilers whine:
src/util/virutil.c: In function 'virMemoryMaxValue':
src/util/virutil.c:2612: error: declaration of 'ulong' shadows a global declaration [-Wshadow]
/usr/include/sys/types.h:151: error: shadowed declaration is here [-Wshadow]
s/ulong/capped/ to work around the problem