Use vmstate_save_state_with_err() to improve error reporting in the
callers and store a reported error under the migration stream. Add
documentation while at it.
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Avihai Horon <avihaih@nvidia.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
3783f814e7