Code review comment for ~raharper/curtin:fix/dname_to_serial_wwn_if_available

I have some questions inline.

One generall question though...
What happens if maas provides a dname for a device that does not have a wwn or id?
Will install fail?

That can be considered "correct" behavior, but I dont know if maas has been
sending dname (or allows the user to do so) in other cases. I'm not sure
that we can start failing an install when previously we just did
the wrong thing and dname didnt work.

Last, we'd like to add some doc on dname. Cove
 - behavior when a name is given to a partition
 - behavior when a name is given to a md or vg