how can i  determining a vm system disk installed on which disk

i get a vm info so how can i get vm system disk installed on whick disk or device
like this

```
content = si.RetrieveContent()
search_index = content.searchIndex
vm = search_index.FindByUuid(None, uuid, True)
vm.layout.disk
vm.config
```
i only get the disk key and device info 
i want get  the vm system disk 
thx for help