spapr/xive: Use device_class_set_parent_realize()

The XIVE router base class currently inherits an empty realize hook
from the sysbus device base class, but it will soon implement one
of its own to perform some sanity checks. Do the preliminary plumbing
to have it called.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20191219181155.32530-6-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>