Comment 14 for bug 1925722

> Here is my updated and untested proposal. I still plan to test this this morning.
> https://pastebin.ubuntu.com/p/DfQqmqDZQJ/
>
> * Some disks may not have serial indicated, and that's OK. Use the
> supplied path instead if present.

I would avoid relaxing this to prevent using the wrong disk. MAAS knows the serial
numbers of the disks.

> * grub_device means we mean to image THAT device, so we shouldn't keep
> the entire list of devices as plausible image targets.

grub_device can be set on more than one disk. In which case we still need to handle the case where we've been given to valid targets and selecting one.

+ if i.get("grub_device"):
+ serial = i.get("serial")
+ if serial:
+ devices = [block.lookup_disk(serial)]
+ LOG.info("choosing device %s based on grub_device flag "
+ "and serial", devices[0])
+ break

This is the general idea; as soon as we meet the selection criteria
we can exit the loop and we have THE specified disk. Note that
block.lookup_disk(serial) may not find the disk in which case it throws
a ValueError; I suggest we continue searching of lookup fails.