xena: Back 2 Back implementation

xena: Back 2 Back implementation

Allows for execution of back 2 back tests by enabling back to back
test scenario in json file when executing Xena2544.exe. Return
results are the number of frames. Results for test execution can
be found in the coresponding JIRA ticket.

- Disabled throughput by default in json file.
- Modified test params in baseconfig file for b2b test
- Modified create_result method to use data in resulting xml file
  over using inspect strategy to detect test type
- Added line to Xena installation guide for issue that was found
  locally after installation

JIRA: VSPERF-264

Change-Id: I0ecd6e1ce750613c53fe308aea3f5a55cca69c20
Signed-off-by: Christian Trautman <ctrautma@redhat.com>