virglrenderer: promote apis required by venus to stable

Let me send this out separately before Venus release.

• bump stable VIRGL_RENDERER_CALLBACKS_VERSION to 3
  • write_context_fence
  • get_server_fd
• promote a couple renderer flags
  • VIRGL_RENDERER_VENUS
  • VIRGL_RENDERER_NO_VIRGL
  • VIRGL_RENDERER_ASYNC_FENCE_CB
  • VIRGL_RENDERER_RENDER_SERVER
• promote 2 renderer apis
  • virgl_renderer_resource_import_blob
  • virgl_renderer_context_create_fence

/cc @ryanneph @robclark @gerddie @tintou @digetx