pcihp: piix4: do not redirect hotplug controller to piix4 when ACPI hotplug is disabled

commit [1] added ability to disable ACPI PCI hotplug
on hostbridge but forgot to take into account that it
should disable all ACPI hotplug machinery in case both
hostbridge and bridge hotplug are disabled.

Commit [2] tried to fix that, however it forgot to
remove hotplug_handler override which hands hotplug
control over to piix4 hotplug controller
(uninitialized after [2]).

As result at the time bridge is plugged in, its default
(SHPC) hotplug handler is replaced by piix4 one in
  acpi_pcihp_device_plug_cb()
    ...
    if (!s->legacy_piix &&
       ...
       qbus_set_hotplug_handler(BUS(sec), OBJECT(hotplug_dev));

which is acting on uninitialized s->legacy_piix value
(0 by default) that was supposed to be initialized by
acpi_pcihp_init(), that is no longer called due to
following condition being false:

  piix4_acpi_system_hot_add_init()
    if (s->use_acpi_hotplug_bridge || s->use_acpi_root_pci_hotplug) {

and the bridge ends up with piix4 as hotplug handler
instead of shpc one.

Followup hotplug on that bridge as result yields
piix4 specific error:

  Error: Unsupported bus. Bus doesn't have property 'acpi-pcihp-bsel' set

1) 3d7e78aa777 (Introduce a new flag for i440fx to disable PCI hotplug on the root bus)
2) df4008c9c59 (piix4: don't reserve hw resources when hotplug is off globally)

Fixes: df4008c9c59 (piix4: don't reserve hw resources when hotplug is off globally)
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20230302161543.286002-12-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/acpi/piix4.c

@@ -492,7 +492,6 @@ static void piix4_pm_realize(PCIDevice *dev, Error **errp)
 
     piix4_acpi_system_hot_add_init(pci_address_space_io(dev),
                                    pci_get_bus(dev), s);
-    qbus_set_hotplug_handler(BUS(pci_get_bus(dev)), OBJECT(s));
 
     piix4_pm_add_properties(s);
 }
@@ -564,6 +563,7 @@ static void piix4_acpi_system_hot_add_init(MemoryRegion *parent,
     if (s->use_acpi_hotplug_bridge || s->use_acpi_root_pci_hotplug) {
         acpi_pcihp_init(OBJECT(s), &s->acpi_pci_hotplug, bus, parent,
                         s->use_acpi_hotplug_bridge, ACPI_PCIHP_ADDR_PIIX4);
+        qbus_set_hotplug_handler(BUS(pci_get_bus(PCI_DEVICE(s))), OBJECT(s));
     }
 
     s->cpu_hotplug_legacy = true;