qemu.tests.netperf_stress: Support multi vms and multi servers/clients.

Signed-off-by: Feng Yang <fyang@redhat.com>
Add multi_vms_netperf_stress test case.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.cfg: update netperf_stress_test.cfg according to new netperf_stress.py script.

Signed-off-by: Feng Yang <fyang@redhat.com>