tools/virtio: option to build an out of tree module

Handy for testing with distro kernels.
Warn that the resulting module is completely unsupported,
and isn't intended for production use.

Usage:
        make oot # builds vhost_test.ko, vhost.ko
        make oot-clean # cleans out files created

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>