x86: Fix cpu online/offline bug: mce memory leak.

x86: Fix cpu online/offline bug: mce memory leak.

Current Xen mce logic didn't free mcabanks. This would be a memory
leak when cpu offline.  When repeatly do cpu online/offline, this
memory leak would make xenpool shrink, and at a time point, will call
alloc_heap_pages --> flush_area_mask, which
ASSERT(local_irq_is_enabled()).  However, cpu online is irq disable,
so it finally result in Xen crash.

This patch fix the memory leak bug, and tested OK over 50,000 round
cpu online/offline.

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>
Signed-off-by: Keir Fraser <keir@xen.org>