qemu.cfg: Update multi_nics_stress.cfg config.

Rename multi_nic_stress.cfg to multi_nics_stress.cfg
Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests.tests: Re-design multi_nics_stress.py script

update this script to match with our test case. Also make it works with
windows guest.
Rename multi_nic_stress.py to multi_nics_stress.py to keep consistent with
configure name.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #185 from FengYang/multi_nics_stress

Multi nics stress