maintainers: Add myself as linux-user reviewer

I volunteer to review linux-user patches.
Adding myself will help to not miss some of them.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Acked-by: Riku Voipio <riku.voipio@linaro.org>
Message-id: 20170510153950.29343-1-laurent@vivier.eu
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>