Xen: reduce memory required for cpu_evtchn_mask
Xen: reduce memory required for cpu_evtchn_mask

Impact: reduce memory usage.

Reduce this significant gain in the amount of memory used
when NR_CPUS bumped from 128 to 4096 by allocating the
array based on nr_cpu_ids:

    65536  +2031616   2097152 +3100%  cpu_evtchn_mask(.bss)

Signed-off-by: Mike Travis <e6cc0fb2b8dad4110ef62e9a33e5a8aa4e0f86d7@sgi.com>
Cc: Jeremy Fitzhardinge <b3f594e10a9edcf5413cf1190121d45078c62290@xensource.com>
Cc: Chris Wright <e829bd48de90b6e1a1950750fa8ca2bb62f273d4@sous-sol.org>
Cc: 65ac89e807f0ba8b231fca45370e2e73a1754d6e@lists.osdl.org
Cc: 196a79ea1bad81d8c954adf793448b0a45442f28@lists.xensource.com

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs