Allow to use nested virtualization again

Current combination of libvirt 1.1.2 and last 0.0.12 `VL` are producing weird errors for me

```
Call to virDomainCreateWithFlags failed: XML error: Non-empty feature list specified without CPU model 
```

There were some discussion about it, but seems it is ok and sane to have not `host-passthrough` but `host-model`

* Cpu will be build from set of options of a host system - this is sane
* requested by @jordant svm will appear in guest if host supports it - sane as well.