MAINTAINERS: Add 9p test client to section "virtio-9p"

MAINTAINERS: Add 9p test client to section "virtio-9p"

The 9p test cases use a dedicated, lite-weight 9p client implementation
(using virtio transport) under tests/qtest/libqos/ to communicate with
QEMU's 9p server.

It's already there for a long time. Let's officially assign it to 9p
maintainers.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Message-Id: <E1ozhlV-0007BU-0g@lizzy.crudebyte.com>