virtio-gpu: qemu 6.1.0 no longer enables virgl when using '-vga virtio'

The problem manifested itself only when inspecting the guest kernel log: [ 1.993537] [drm] features: -virgl +edid -resource_blob -host_visible

After much digging through the code, I realized this was probably lost in the recent virtio-gpu refactoring: https://www.kraxel.org/blog/2021/05/virtio-gpu-qemu-graphics-update/

'-device virtio-vga-gl' works, but isn't obvious.