backends/drm: support atomic modesetting in virtual machines

With kernel 6.6+, HOTSPOT_X and HOTSPOT_Y are exposed on the cursor plane in virtual machines, so the fallback to the legacy API is no longer needed. At some point in the future, the list of virtual machine drivers can be dropped entirely.

This doesn't really need to go into 6 as it doesn't fix any bugs I know of, but it's quite small and it would be nice to move away from special casing every driver (and from legacy in general).