sysctl: make XEN_SYSCTL_numainfo a little more efficient

sysctl: make XEN_SYSCTL_numainfo a little more efficient

A number of changes to XEN_SYSCTL_numainfo interface:

* Make sysctl NUMA topology query use fewer copies by combining some
  fields into a single structure and copying distances for each node
  in a single copy.
* NULL meminfo and distance handles are a request for maximum number
  of nodes (num_nodes). If those handles are valid and num_nodes is
  is smaller than the number of nodes in the system then -ENOBUFS is
  returned (and correct num_nodes is provided)
* Instead of using max_node_index for passing number of nodes keep this
  value in num_nodes: almost all uses of max_node_index required adding
  or subtracting one to eventually get to number of nodes anyway.
* Replace INVALID_NUMAINFO_ID with XEN_INVALID_MEM_SZ and add
  XEN_INVALID_NODE_DIST.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>