Support adding host devices to instances

I want to capture the video from the web usb camera of multipass instance, but I can not
find how to add the video device to multipass instance.

Before use multipass, I had use docker container.
Under the docker container, I can add video device just like: docker run -d /dev/video0:/dev/video0 ........

The issue mentions no files, tests, or entry points. Start by locating Multipass's instance device and host integration code, then determine the supported host platforms and device-passthrough requirements. Done means a host video device can be exposed reliably inside an instance, with coverage for the supported environments.

Written by the indexing model from the issue text.