fix(disk) preserve devices.root.size.state key on instance configuration update

[edlerd]

Done

• preserve devices.root.size.state key on instance configuration update

Fixes WD-13755

QA

1. Run the LXD-UI:
   • On the demo server via the link posted by @webteam-app below. This is only available for PRs created by collaborators of the repo. Ask @mas-who or @edlerd for access.
   • With a local copy of this branch, build and run as described in the docs.
2. Perform the following QA steps:
   • create an instance with the devices.root.size.state key set i.e. to 4GiB
   • edit instance configuration and save
   • ensure the key is persisted on save.

[webteam-app]

Demo

Jenkins

demos.haus

[mas-who]

LGTM, thanks for the fix 👍