add virgl_renderer_context_create_with_capset (and vtest changes)
This MR adds virgl_renderer_context_create_with_capset
and corresponding vtest changes.
All user-visible changes are guarded by VIRGL_RENDERER_UNSTABLE_APIS
because they are only verified by vtest. They will become stable only after the corresponding changes for VMMs/virtio-gpu/kernel are landed.