In commit baf86d6b we switched the default trace backend from "nop"
to "log". Update the documentation to match.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1478276837-31780-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>