vhost-user: unregister slave req handler at cleanup time

If the backend sends a request just before closing the socket,
the aio dispatcher might schedule its reading after the vhost
device has been cleaned, leading to a NULL pointer dereference
in slave_read();

vhost_user_cleanup() already closes the socket but it is not
enough, the handler has to be unregistered.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
