spapr_pci: fail gracefully with non-pseries machine types

QEMU currently crashes when the user tries to add an spapr-pci-host-bridge
on a non-pseries machine:

$ qemu-system-ppc64 -M ppce500 -device spapr-pci-host-bridge,index=1
hw/ppc/spapr_pci.c:1535:spapr_phb_realize:
Object 0x1003dacae60 is not an instance of type spapr-machine
Aborted (core dumped)

The same thing happens with the deprecated but still available child type
spapr-pci-vfio-host-bridge.

Fix both by checking the machine type with object_dynamic_cast().
Reviewed-by: NDaniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Signed-off-by: NGreg Kurz <groug@kaod.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr_pci.c

@@ -1507,7 +1507,12 @@ static void spapr_pci_unplug_request(HotplugHandler *plug_handler,
 
 static void spapr_phb_realize(DeviceState *dev, Error **errp)
 {
-    sPAPRMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
+    /* We don't use SPAPR_MACHINE() in order to exit gracefully if the user
+     * tries to add a sPAPR PHB to a non-pseries machine.
+     */
+    sPAPRMachineState *spapr =
+        (sPAPRMachineState *) object_dynamic_cast(qdev_get_machine(),
+                                                  TYPE_SPAPR_MACHINE);
     SysBusDevice *s = SYS_BUS_DEVICE(dev);
     sPAPRPHBState *sphb = SPAPR_PCI_HOST_BRIDGE(s);
     PCIHostState *phb = PCI_HOST_BRIDGE(s);
@@ -1519,6 +1524,11 @@ static void spapr_phb_realize(DeviceState *dev, Error **errp)
     const unsigned windows_supported =
         sphb->ddw_enabled ? SPAPR_PCI_DMA_MAX_WINDOWS : 1;
 
+    if (!spapr) {
+        error_setg(errp, TYPE_SPAPR_PCI_HOST_BRIDGE " needs a pseries machine");
+        return;
+    }
+
     if (sphb->index != (uint32_t)-1) {
         sPAPRMachineClass *smc = SPAPR_MACHINE_GET_CLASS(spapr);
         Error *local_err = NULL;