oeqa/qemurunner: Use oe._exit(), not sys.exit()

sys.exit will cause finally statements and other code to run at exit. Since
we're using os.fork() here, os._exit() is apprioriate in this codepath.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ec08498f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>