vdrm: vtest support

Adds a vtest backend to vdrm, and add turnip support to use it.

With this, we cannot do direct syncobj ioctl calls in common vk code, so a util_sync_provider shim is added. When vtest is used the syncobj ioctls are proxied over to the vtest server. (Blocking calls are implemented by the server passing back a pipe fd that mesa can read from.)

virglrenderer part: virgl/virglrenderer!805