ntb_perf: extend with poll latency measurement

Poll latency is a delay between start to send 1 byte to the
remote system and receiving the confirmation. The remote system needs to
be run in server mode beforehand. Then the server polls the input buffer
and on receiving data immediately sends the confirmation back.

Signed-off-by: Alexander Fomichev <a.fomichev@yadro.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>