Add setters for `name_label` `name_description`

[djs55]

We can set a name/description in Volume.create but we can't

• update these later
• select new values for a clone or snapshot (this is useful to mark the created Volume to avoid having to run a GC later if the client fails between the clone and set calls)

[gaborigloi]

These have been added: https://github.com/xapi-project/xapi-storage/blob/master/generator/lib/control.ml#L217