sflow: Export LAG, PORTNAME, and OPENFLOWPORT information also.

Export standard sFlow LAG, PORTNAME and OPENFLOWPORT structures with each
counter-sample. Add unit-test for sFlow-LAG. Adjust other unit-tests to
accommodate these new annotations.

The sFlow-LAG structures are important for topology discovery, for
troubleshooting LAG instability,  and for correctly combining
sFlow feeds from multiple sources.

The OPENFLOWPORT and PORTNAME structures are important for systems that
aim to combine sFlow monitoring with OpenFlow controls,  as they
provide straightforward mapping (1) between sFlow agent IP and OpenFlow
datapath-id,  and (2) between interface name,ifIndex and OpenFlow
port number.

Signed-off-by: Neil McKee <neil.mckee@inmon.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>