With EFI and simpledrm we may end up booting without a card0 but with a card1 drm device. This defeats GDM's simplistic hybrid graphics check.
Also, our detection for virtio graphics wasn't working so well, because it wrote the results into an environment variable of a non-parent device.
This commit gets the detection logic working better for virt/efi setups.
Closes: #832 (closed)