oeqa/qemurunner: add run_serial() comment

Add a comment explaining the non-obvious return codes.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6572baff)
Signed-off-by: Steve Sakoman <steve@sakoman.com>