Feature request: filter by subsystem for unix-hotplug devices

[jonathan-conder]

Currently unix-hotplug devices can be detected based on productid and vendorid. It would be great if this could be extended to include subsystem as well.

For example, it would be really convenient to allow containers to access all camera devices with a single command:

lxc config device add u1 cameras unix-hotplug subsystem=video4linux

Or all cameras with a specific vendorid, and so on:

lxc config device add u1 logitechcameras unix-hotplug subsystem=video4linux vendorid=****

Some devices don't currently have product and vendor IDs, so this change would enable those as well. For example fake v4l2loopback devices are useful for testing. My camera also exposes /dev/media0, which belongs to the media subsystem but lacks a product/vendor ID.