Revert "s390x/ccw: create s390 phb conditionally"

This reverts commit d32bd03.

Turns out that old QEMUs always created a pci host bridge
and for many CPU models the migration from old QEMUs to new
QEMUs will fail with
qemu-system-s390x: Unknown savevm section or instance 'PCIBUS' 0
qemu-system-s390x: load of migration failed: Invalid argument

As a quick fix we will revert the commit and always create the
pci host bridge.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
[fixed revert to keep the comment fixup, added a comment in the code]
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Message-Id: <20170928131831.81393-1-borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

hw/s390x/s390-virtio-ccw.c

@@ -251,6 +251,7 @@ static void ccw_init(MachineState *machine)
 {
     int ret;
     VirtualCssBus *css_bus;
+    DeviceState *dev;
 
     s390_sclp_init();
     s390_memory_init(machine->ram_size);
@@ -266,13 +267,14 @@ static void ccw_init(MachineState *machine)
                       machine->initrd_filename, "s390-ccw.img",
                       "s390-netboot.img", true);
 
-    if (s390_has_feat(S390_FEAT_ZPCI)) {
-        DeviceState *dev = qdev_create(NULL, TYPE_S390_PCI_HOST_BRIDGE);
-        object_property_add_child(qdev_get_machine(),
-                                  TYPE_S390_PCI_HOST_BRIDGE,
-                                  OBJECT(dev), NULL);
-        qdev_init_nofail(dev);
-    }
+    /*
+     * We cannot easily make the pci host bridge conditional as older QEMUs
+     * always created it. Doing so would break migration across QEMU versions.
+     */
+    dev = qdev_create(NULL, TYPE_S390_PCI_HOST_BRIDGE);
+    object_property_add_child(qdev_get_machine(), TYPE_S390_PCI_HOST_BRIDGE,
+                              OBJECT(dev), NULL);
+    qdev_init_nofail(dev);
 
     /* register hypercalls */
     virtio_ccw_register_hcalls();