virgl: move protocol headers to a common place
This moves protocol headers found in src/gallium/drivers/virgl and src/gallium/winsys/virgl/vtest to src/virtio/{virgl,vtest} respectively. This allows the headers to be shared when a vulkan driver is added for example.
It also updates the headers to the latest ones from virglrenderer. There are some new commands that I am not sure what their status are. If they are unstable, they should be fixed (in upstream) such that they are guarded by VIRGL_RENDERER_UNSTABLE_APIS.