Description

Fixes #1046, at least the bit about USB devices being completely skipped. It still won't support hotplugging or ownership inheritance until those are fixed in LXD (canonical/lxd#19450, canonical/lxd#19451).

LXD deliberately filters out subsystem: usb when starting a unix-hotplug device, because it has a dedicated usb device type. Workshop now uses both unix-hotplug and usb devices to back a custom-device plug when subsystem is unset. When subsystem=usb it only uses a usb device, no unix-hotplug.

Self-review quick check

• Make decisions that cost a lot to reverse explicit in the PR description.
• Avoid nested conditions.
• Delete dead code and redundant comments.
• Normalise symmetries by sticking to doing identical things identically.

// one way to handle errors
if err := f(); err != nil {
   ...
}

// one way to handle multiple returns
val, err := f()
if err != nil {
   ...
}
...

• Check that coupled code elements, files, and directories are adjacent. For example, test data is stored as close as possible to a test.
• Put variable declaration and initialisation together.
• Divide large expressions into digestable and self-explanatory ones. Use multiple variables if required.
• Put a blank line between two logically different chunks of code.
• Follow the style guide for new error messages.

Docs

Procedure:

• I have checked and added or updated relevant documentation.
• I have checked and added or updated relevant release notes.
• I have included the technical author in the review.

Content:

• Headings and titles accurately describe the content.
• New and updated pages include correct metadata.
• Documentation tests are added or updated where applicable (for tutorial/ and how-to/ sections).
• Documentation follows the style guide.
• If needed, docs/.coverage.yaml updated, coverage tags added (.. artefact).

Or:

• I confirm the PR has no implications for documentation.