This helper is similar to v9fs_string_sprintf(), but it includes the
terminating NUL character in the size field.

This is to avoid doing v9fs_string_sprintf((V9fsString *) &path) and
then bumping the size.

Affected users are changed to use this new helper.
Signed-off-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NCédric Le Goater <clg@kaod.org>