trex: RFC2544 Throughput bugfixing

trex: RFC2544 Throughput bugfixing

Changes introduced by patch:
* Ensure that correct results with given lossreate are returned
* Add threshold value to specify the accurancy of RFC2544 Throughput
  measurement. Default value is 0.05.

JIRA: VSPERF-530

Change-Id: I8148deba9039c3a0feb6394d6671aa10c5afaf0a
Signed-off-by: Martin Goldammer <martin.goldammer6@gmail.com>
Reviewed-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>