tests: Update the version command in netperf

This will help to remove the blank lines in the results file. And
make the format suitable for further result analyze.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>
tests.netperf: Add some string process code

Process some output value before use them to igonre the problem caused
by different guest os version and session setup.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>