qemu.tests.migration_with_netperf: Support netperf client in guest

Add netperf client in guest. Now start netperf and netserver both in
guest and host.

Add netperf.exe in windows guest.

Please put ftp://ftp.netperf.org/netperf/misc/netperf.exe under top
deps folder.
Please also put netperf and netserver.exe in same folder.

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