Draft: Run Virgl tests with the virtio-gpu winsys
Add a first test job that exercises the virtio-gpu winsys in Virgl, using boards that support KVM.
For this, I'm using Crosvm which is leaner than QEMU and also happens to be the VMM used in one of the biggest Virgl deployments.
The root filesystem is mounted read-only into the guest, so I had to make some changes related to pats of temporary files.