spapr/xive: fix multiple resets when using the 'dual' interrupt mode

Today, when a reset occurs on a pseries machine using the 'dual'
interrupt mode, the KVM devices are released and recreated depending
on the interrupt mode selected by CAS. If XIVE is selected, the SysBus
memory regions of the SpaprXive model are initialized by the KVM
backend initialization routine each time a reset occurs. This leads to
a crash after a couple of resets because the machine reaches the
QDEV_MAX_MMIO limit of SysBusDevice :

qemu-system-ppc64: hw/core/sysbus.c:193: sysbus_init_mmio: Assertion `dev->num_mmio < QDEV_MAX_MMIO' failed.

To fix, initialize the SysBus memory regions in spapr_xive_realize()
called only once and remove the same inits from the QEMU and KVM
backend initialization routines which are called at each reset.

Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190522074016.10521-2-clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>