[XEN] Add arch hook for max_mem domctl.

[XEN] Add arch hook for max_mem domctl.

This patch introduces an arch hook in the MAX_MEM hcall. In particular, PowerPC
will use this hook to create/update a p2m mapping array. This patch introduces
the hook only and should compile away on all arches.

Signed-off-by: Ryan Harper <ryanh@us.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
--HG--
extra : transplant_source : %BD%0B%97%A5i%FE%9F%5C%99%C2%01S%80%B6%C0%F0%82_l%A3