interacting with libvirt outside of nixops is source of bugs

I removed some domains via virsh for some tests and when I try to remove it via nixops afterwards, it triggers:
```
server> Warning: Domaine non trouvé : no domain with matching name 'nixops-34c11b0f-d3a4-11e7-9ebd-309c233b770e-server'
error: 'NoneType' object has no attribute 'isActive'
```
there should also be a --force to destroy for similar cases 
