nicira-ext: Separate nx_aggregate_stats_reply from ofp_aggregate_stats_reply.

nicira-ext: Separate nx_aggregate_stats_reply from ofp_aggregate_stats_reply.

An upcoming patch will make stats messages much more like other messages,
in that their structures will include all of the headers.  This means that
struct ofp_aggregate_stats_reply will no longer be appropriate as a
member of struct nx_aggregate_stats_reply, because it will then include
those additional header members.

Also, struct nx_aggregate_stats_reply doesn't need to use the special
ovs_32aligned_be64 type, since its 64-bit members are correctly aligned,
which is another reasonable reason to keep it separate.