drm/virtio: Spiff out cmd queue/response traces

Add a sequence # for more easily matching up cmd/resp, and the # of free
slots in the virtqueue to more easily see starvation issues.

v2: Fix handling of string fields as well

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221130000841.318037-1-robdclark@gmail.com