lxd/device: unix-hotplug ownership fix

[kadinsayani]

Fixes the second bug raised in #14266.

This PR adds a setting, ownership.inherit for unix-hotplug devices and a function unixDeviceOwnership() which returns device ownership.

New behaviour:

• The default setting for ownership.inherit is false;
• When ownership.inherit is set to true, the device ownership is inherited from the host;
• ownership.inherit cannot be set to true when gid or uid are set.

Existing behaviour (unchanged):

• When gid and uid are set, they are used for device ownership;
• If gid and uid are not present in the config, root (0) ownership is used.

[tomponline]

@kadinsayani please can you update the PR description to explain the change of behaviour in this PR and what, if any, will change for existing users?

[kadinsayani]

@kadinsayani please can you update the PR description to explain the change of behaviour in this PR and what, if any, will change for existing users?

Sure thing!

[github-actions]

Heads up @mionaalex - the "Documentation" label was applied to this issue.

[kadinsayani]

I noticed some wording inconsistencies in the documentation for unix-hotplug and the other unix device types. I've opened #14451 to track the issue and engage @minaelee for feedback. I'll open up a separate PR to update the docs and resolve the issue.

[kadinsayani]

@tomponline, do we need an API extension for the new setting even if we're not adding tests?