x86: Increase the default NR_CPUS to 256

Changeset 21012:ef845a385014 bumped the default to 128 about one and a half years ago. Increase it now to 256, as systems with eg. 160 logical CPUs are becoming (have become) common.

Thanks for considering.

diff -r 227130622561 xen/include/asm-x86/config.h
@@ -50,7 +50,7 @@
 #ifdef MAX_PHYS_CPUS
 #define NR_CPUS MAX_PHYS_CPUS
 #else
-#define NR_CPUS 128
+#define NR_CPUS 256
 #endif
 
 #ifdef __i386__