xend: Fix bug of cpu affinity/ vcpu pin under ia32pae

xend: Fix bug of cpu affinity/ vcpu pin under ia32pae

c/s 21040 and 21044 used to break cpu number limit (<=64).
However, they result in bug under ia32pae model:
1. things will go wrong for affinity, making all vcpus pin to a same
cpu with same time;
2. when 'xm vcpu-pin' vpu to cpu, xend will exit abnormally.

This patch is to fix the bugs described above.

Signed-off-by: Xu, Jiajun <jiajun.xu@intel.com>
Signed-off-by: Zhang, Jianwu <jianwu.zhang@inte.com>
Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>