qemu.tests: new case to test quit vm in openvswith bridge

New case to test  tap device auto delete from openvswith bridge when qemu-kvm quit with error.  Before start test, please update switch name in files 'deps/ovs/qemu-ifup' and 'deps/ovs/qemu-ifup' with real bridge name in your system or special 'nic_script' and 'nic_downscript'  params in your test configure file.

Thanks,
Xu
Why need to boot three vms? And i don't see the "vms =" config in your config file.