Filter volume snapshots on selecting custom volume

[edlerd]

Done

• Filter volume snapshots on selecting custom volume to add to an instance or profile. only show the custom volumes to select from.

Fixes #777

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, run as described here.
2. Perform the following QA steps:
   • Create a custom storage volume
   • Create snapshots of the volume
   • Go to configure instance, add disk device, select volume. Ensure the storage volume is available, but the snapshot is not.

[webteam-app]

Demo

Jenkins

demos.haus

[mas-who]

LGTM, just the sign-off is missing.