thunderbolt: Build initial XDomain property block upon first connect

On a systems where the Thunderbolt controller is present all the time
the kernel nodename may not yet set by the userspace when the driver is
loaded. This means when another host is connected it may see the default
"(none)" hostname instead of the system real hostname.

For this reason build the initial XDomain property block only upon first
connect. This should make sure the userspace has had chance to set it up.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>