Libvirt.tests: Migration tests' enhancements

1.Parameter vm_ref should be setmmdt_vm_ref in setmaxdowntime tests.
2.Problems in migrate stress tests:
```
 * Vms will be popped in cross migration, so creating them again
   for cleanup.
 * If local vms use lan, geting address would be failed on remote
   host, so storing ips before migration.
 * Fix stress_args parameter.
```