spapr, pnv, xive: Add a "xive-fabric" link to the XIVE router

In order to get rid of qdev_get_machine(), first add a pointer to the
XIVE fabric under the XIVE router and make it configurable through a
QOM link property.

Configure it in the spapr and pnv machine. In the case of pnv, the XIVE
routers are under the chip, so this is done with a QOM alias property of
the POWER9 pnv chip.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20200106145645.4539-5-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>