thunderbolt: Create XDomain devices for loops back to the host

It is perfectly possible to have loops back from the routers to the
host, or even from one host port to another. Instead of ignoring these,
we create XDomain devices for each. This allows creating services such
as DMA traffic test that is used in manufacturing for example.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>