some ideas for multi gpu and vfio

(1) if the primary GPU has no outputs connected but we have a secondary one with outputs, automatically select that secondary one to improve performance
(2) dynamically do (1) when the user hot(un)plugs monitors, and switch back to the boot vga if it gets an output again
(3) automatically prefer external GPUs if they have outputs connected: external GPUs are much more affected by copying images back and forth
(4) if the primary gpu gets unplugged, switch to some other gpu if possible instead of exiting
(5) if the last remaining gpu gets unplugged, make it possible to not exit but instead wait for the gpu to get hotplugged again. This would allow for single-gpu VFIO without killing the session, assuming that clients support it eventually
(6) offer an interface (in kscreen) that allows a user to make KWin stop using a GPU (temporarily) to allow for easier hot-unplugging with VFIO
(7) allow the user to configure their primary GPU in a kcm, perhaps like the output properties per display arrangement

A few of these things depend on finding out which render node Mesa uses for a given KMS device (which is also required for wp_linux_dmabuf_hints) and whether or not a GPU is external. (1), (2) and (4) can be done pretty easily though