virgl: forward the host renderer hardware info

Some game engines rely on the real hardware info to adjust default graphics quality and other attributes.

This patch also prepends "virgl" to the forwarded host renderer string. This avoids breaking app compat while allowing game engines to easily fix up the rendering default attributes. Final format string: "virgl (%s)"

virglrenderer MR: virgl/virglrenderer!532 (merged)

/cc @olv