ppc/pnv: Make PSI device types not user creatable

QEMU aborts with -device pnv-psi-POWER8:

$ qemu-system-ppc64 -device pnv-psi-POWER8
qemu-system-ppc64: hw/intc/xics.c:605: ics_realize: Assertion
`ics->xics' failed.
Aborted (core dumped)

The Processor Service Interface Controller is an internal device.
It should only be instantiated by the chip, which takes care of
configuring the link required by the ICS object in the case of
POWER8. It doesn't make sense for a user to specify it on the
command line.

Note that the PSI model for POWER8 was added 3 yrs ago but the
devices weren't available on the command line because of a bug
that was fixed by recent commit 2f35254a ("pnv/psi: Correct
the pnv-psi* devices not to be sysbus devices").

Fixes: 54f59d78 ("ppc/pnv: Add cut down PSI bridge model and hookup external interrupt")
Reported-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NGreg Kurz <groug@kaod.org>
Message-Id: <159413975752.169116.5808968580649255382.stgit@bahia.lan>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/pnv_psi.c

@@ -929,6 +929,7 @@ static void pnv_psi_class_init(ObjectClass *klass, void *data)
     dc->desc = "PowerNV PSI Controller";
     device_class_set_props(dc, pnv_psi_properties);
     dc->reset = pnv_psi_reset;
+    dc->user_creatable = false;
 }
 
 static const TypeInfo pnv_psi_info = {