libvirt plugin: Add "PluginInstanceFormat" option.

This commit adds a ```PluginInstanceFormat``` option that, as ```HostnameFormat``` do, accepts several values within ```uuid``` or ```name```.

This allow a configuration as this

```
<Plugin libvirt>
[...]
HostnameFormat       "hostname"
PluginInstanceFormat "name"
[...]
</Plugin>
```

that enable the plugin to create RRD files as ```hypervisor_machine/libvirt-instance-XXXXXX```