MAINTAINERS: Add subprojects/libvhost-user to section "vhost"

Signed-off-by: Stefan Weil <sw@weilnetz.de>
[Michael agreed to act as maintainer for libvhost-user via email in
https://lore.kernel.org/qemu-devel/20221123015218-mutt-send-email-mst@kernel.org/.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221126152507.283271-6-sw@weilnetz.de>

MAINTAINERS

@@ -2005,6 +2005,7 @@ F: docs/interop/vhost-user.rst
 F: contrib/vhost-user-*/
 F: backends/vhost-user.c
 F: include/sysemu/vhost-user-backend.h
+F: subprojects/libvhost-user/
 
 virtio
 M: Michael S. Tsirkin <mst@redhat.com>