Use g_autofree to ensure that `config` is freed when
vhost_vdpa_get_max_queue_pairs() returns.

Reported-by: Coverity (CID 1465228: RESOURCE_LEAK)
Fixes: 40237840 ("vhost-vdpa: multiqueue support")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20211102155157.241034-1-sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>