force delete VM with existing snapshots

> Requested operation is not valid: cannot delete inactive domain with 1 snapshots

Deleting the snapshots manually before being able to delete the VM doesn't much make sense. ;)

But what's more important, external snapshots (not supported by WVM itself) can't be deleted ([libvirt #1044691](https://bugzilla.redhat.com/show_bug.cgi?id=1044691#c11)). That said, it's inevitable to pass `VIR_DOMAIN_UNDEFINE_SNAPSHOTS_METADATA` to successfully delete a VM.
I would enclose this in a try - except in the case flag is not supported by installed libvirtd version

```python
    def delete(self):
        try:
            self.instance.undefineFlags(VIR_DOMAIN_UNDEFINE_SNAPSHOTS_METADATA)
        except:
            self.instance.undefine()
```