UPSTREAM: virtio-gpu api: comment feature flags

Add comments to the existing feature flags,
documenting which commands belong to them.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190410114227.25846-2-kraxel@redhat.com


(cherry picked from commit beb941b970fb129dae206050593d3e768859b146)
Bug: 153580313
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: I8130b6a04aea46f8f6ea8f4ab14acccf0c1932eb