[XEN] NUMA-ify Xen heap and page allocator

[XEN] NUMA-ify Xen heap and page allocator

This patch adds a per-node bucket to the heap structure in Xen.
During heap initialization the patch determines which bucket to place
the memory. We reserve guard pages between node boundaries in the case
that said boundary isn't already guarded by the MAX_ORDER boundary to
prevent the buddy allocator from merging pages between nodes.

Signed-off-by: Ryan Harper <ryanh@us.ibm.com>