hw/pxb-pcie: fix PCI Express hotplug support

Add the missing osc method for pxb-pcie devices as APCI spec recommends,
see 6.2.9.1 OSC Implementation Example for PCI Host Bridge Devices, ACPI 3.0a:

    It is recommended that a machine with multiple host bridge devices
    should report the same capabilities for all host bridges, and also
    negotiate control of the features described in the Control Field in
    the same way for all host bridges.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/i386/acpi-build.c

@@ -1962,6 +1962,9 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
             aml_append(dev, aml_name_decl("_UID", aml_int(bus_num)));
             aml_append(dev, aml_name_decl("_HID", aml_eisaid("PNP0A03")));
             aml_append(dev, aml_name_decl("_BBN", aml_int(bus_num)));
+            if (pci_bus_is_express(bus)) {
+                aml_append(dev, build_q35_osc_method());
+            }
 
             if (numa_node != NUMA_NODE_UNASSIGNED) {
                 aml_append(dev, aml_name_decl("_PXM", aml_int(numa_node)));