MAINTAINERS: Upgrade myself as 9pfs co-maintainer

As suggested by Greg, let's upgrade myself as co-maintainer of 9pfs.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <E1jEYz4-0004pt-Cs@lizzy.crudebyte.com>
Signed-off-by: Greg Kurz <groug@kaod.org>

MAINTAINERS

@@ -1657,7 +1657,7 @@ F: include/sysemu/balloon.h
 
 virtio-9p
 M: Greg Kurz <groug@kaod.org>
-R: Christian Schoenebeck <qemu_oss@crudebyte.com>
+M: Christian Schoenebeck <qemu_oss@crudebyte.com>
 S: Odd Fixes
 F: hw/9pfs/
 X: hw/9pfs/xen-9p*