Submit in custom disk device creation modal improvements

[edlerd]

Done

• improve behaviour of submit in custom disk device creation modal for the instance configuration

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 @lorumic or @edlerd for access.
   • With a local copy of this branch, run as described here.
2. Perform the following QA steps:
   • edit or create an instance
   • select disk devices > Attach disk device > Create volume
   • ensure the creation and selection works.
   • The "Create volume" form should render the create button busy after submit (used to be no indicator while loading)
   • Coming back to the "Choose custom volume" should
     • render the new volume on top (used to be in the middle, now sorted by most recent created)
     • initially show a loading state until the new volume is included (used to be hidden and updating in the background at some time)

[webteam-app]

Demo starting at https://lxd-ui-544.demos.haus

[piperdeck]

• In the volume creation modal, the "storage pool" field seems to appear twice

Otherwise, this looks good to me!

[edlerd]

• In the volume creation modal, the "storage pool" field seems to appear twice

Whoopsie, I think this is from a bad merge due to other PRs happening in parallel. Now it should be fine. Please have another pass at this PR @piperdeck

[piperdeck]

LGTM

[mas-who]

QA looks good! Two comments, not really critical so feel free to disregard them.