[websocket-api] NUMA node data for logical volumes

In the latest designs for surfacing NUMA node information in the UI, the Machine details > Storage tab shows the following: https://app.zeplin.io/project/5d6e416f2b1b2b1a84bb820e/screen/5d7a47406ac5c3603a20b070

Physical volumes are simple enough and the current API returns the NUMA node data correctly. However, volume groups, cache sets etc. do not return any NUMA node info and there is no way (as far as I can tell) to link a "parent" storage element to its "children". Therefore we cannot currently build a list of NUMA nodes associated with logical volumes.

I think this could be solved in a couple of ways:
1. Calculate NUMA node info for logical volumes in the backend and send it through the websocket directly, or;
2. Send the indices of the parents/children of volumes over the websocket (which is what currently happens with interfaces) and the list of NUMA nodes could then be calculated on the frontend.