pc: Support coldplugging of virtio-pmem-pci devices on all buses

E.g., with "pc-q35-4.2", trying to coldplug a virtio-pmem-pci devices
results in
    "virtio-pmem-pci not supported on this bus"

Reasons is, that the bus does not support hotplug and, therefore, does
not have a hotplug handler. Let's allow coldplugging virtio-pmem devices
on such buses. The hotplug order is only relevant for virtio-pmem-pci
when the guest is already alive and the device is visible before
memory_device_plug() wired up the memory device bits.

Hotplug attempts will still fail with:
    "Error: Bus 'pcie.0' does not support hotplugging"

Hotunplug attempts will still fail with:
    "Error: Bus 'pcie.0' does not support hotplugging"
Reported-by: NVivek Goyal <vgoyal@redhat.com>
Reviewed-by: NPankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: NDavid Hildenbrand <david@redhat.com>
Message-Id: <20200626072248.78761-2-david@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/i386/pc.c

@@ -1643,13 +1643,13 @@ static void pc_virtio_pmem_pci_pre_plug(HotplugHandler *hotplug_dev,
     HotplugHandler *hotplug_dev2 = qdev_get_bus_hotplug_handler(dev);
     Error *local_err = NULL;
 
-    if (!hotplug_dev2) {
+    if (!hotplug_dev2 && dev->hotplugged) {
         /*
          * Without a bus hotplug handler, we cannot control the plug/unplug
-         * order. This should never be the case on x86, however better add
-         * a safety net.
+         * order. We should never reach this point when hotplugging on x86,
+         * however, better add a safety net.
          */
-        error_setg(errp, "virtio-pmem-pci not supported on this bus.");
+        error_setg(errp, "virtio-pmem-pci hotplug not supported on this bus.");
         return;
     }
     /*
@@ -1658,7 +1658,7 @@ static void pc_virtio_pmem_pci_pre_plug(HotplugHandler *hotplug_dev,
      */
     memory_device_pre_plug(MEMORY_DEVICE(dev), MACHINE(hotplug_dev), NULL,
                            &local_err);
-    if (!local_err) {
+    if (!local_err && hotplug_dev2) {
         hotplug_handler_pre_plug(hotplug_dev2, dev, &local_err);
     }
     error_propagate(errp, local_err);
@@ -1676,9 +1676,11 @@ static void pc_virtio_pmem_pci_plug(HotplugHandler *hotplug_dev,
      * device bits.
      */
     memory_device_plug(MEMORY_DEVICE(dev), MACHINE(hotplug_dev));
-    hotplug_handler_plug(hotplug_dev2, dev, &local_err);
-    if (local_err) {
-        memory_device_unplug(MEMORY_DEVICE(dev), MACHINE(hotplug_dev));
+    if (hotplug_dev2) {
+        hotplug_handler_plug(hotplug_dev2, dev, &local_err);
+        if (local_err) {
+            memory_device_unplug(MEMORY_DEVICE(dev), MACHINE(hotplug_dev));
+        }
     }
     error_propagate(errp, local_err);
 }