xive: Use the XIVE fabric link under the XIVE router

Now that the spapr and pnv machines do set the "xive-fabric" link, the
use of the XIVE fabric pointer becomes mandatory. This is checked with
an assert() in a new realize hook. Since the XIVE router is realized at
machine init for the all the machine's life time, no risk to abort an
already running guest (ie. not a hotplug path).

This gets rid of a qdev_get_machine() call.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20200106145645.4539-6-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>