MAINTAINERS: Change maintership of Xen code under hw/9pfs

Xen folks are the actual maintainers for this.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Message-Id: <155912548463.2019004.3515830305299809902.stgit@bahia.lan>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>