[XEN] Make memory hypercalls NUMA-aware.

[XEN] Make memory hypercalls NUMA-aware.

This patch modifies memory ops to use the NUMA-aware page allocator
functions.  We use the target domain's VCPU0 placement to determine
which node's memory to use.  We expect the system administrator to
utilize the exposed NUMA topology information to help craft guest
config files that are NUMA-friendly (use only processors and memory values
that will fit within a given node).

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