libvirt.tests: Add migration test for virsh_migrate_stress.

Enhancements:
```
* Support migration in thread for expanding.
* Fix bugs and some pylint problems.
```

Cross migration:
```
local     =====      remote
1st time:
vm1        ->
2nd time:
             <-         vm1
vm2        ->
3rd time:
             <-         vm2
vm3        ->
```

Booting migration:
Migrate vms when they are booting(Not load vms).