MAINTAINERS: update email address for Greg Kurz

While here, also add a section for the tree I use for 9p.

Signed-off-by: Greg Kurz <groug@kaod.org>
Acked-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Cédric Le Goater <clg@fr.ibm.com>
Acked-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Message-id: 146617410554.7281.1733165006203821878.stgit@bahia.lan
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>