virtio-pci: add device_unplugged callback

This fixes a crash in hot-unplug of virtio-pci devices behind a PCIe
switch.  The crash happens because the ioeventfd is still set whent the
child is destroyed (destruction happens in postorder).  Then the proxy
tries to unset to ioeventfd, but the virtqueue structure that holds the
EventNotifier has been trashed in the meanwhile.  kvm_set_ioeventfd_pio
does not expect failure and aborts.

The fix is simply to move parts of uninitialization to a new
device_unplugged callback, which is called before the child is destroyed.

Cc: qemu-stable@nongnu.org
Acked-by: NAndreas Faerber <afaerber@suse.de>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/virtio/virtio-pci.c

@@ -1002,6 +1002,15 @@ static void virtio_pci_device_plugged(DeviceState *d)
                                                       proxy->host_features);
 }
 
+static void virtio_pci_device_unplugged(DeviceState *d)
+{
+    PCIDevice *pci_dev = PCI_DEVICE(d);
+    VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
+
+    virtio_pci_stop_ioeventfd(proxy);
+    msix_uninit_exclusive_bar(pci_dev);
+}
+
 static int virtio_pci_init(PCIDevice *pci_dev)
 {
     VirtIOPCIProxy *dev = VIRTIO_PCI(pci_dev);
@@ -1016,9 +1025,7 @@ static int virtio_pci_init(PCIDevice *pci_dev)
 static void virtio_pci_exit(PCIDevice *pci_dev)
 {
     VirtIOPCIProxy *proxy = VIRTIO_PCI(pci_dev);
-    virtio_pci_stop_ioeventfd(proxy);
     memory_region_destroy(&proxy->bar);
-    msix_uninit_exclusive_bar(pci_dev);
 }
 
 static void virtio_pci_reset(DeviceState *qdev)
@@ -1553,6 +1560,7 @@ static void virtio_pci_bus_class_init(ObjectClass *klass, void *data)
     k->set_guest_notifiers = virtio_pci_set_guest_notifiers;
     k->vmstate_change = virtio_pci_vmstate_change;
     k->device_plugged = virtio_pci_device_plugged;
+    k->device_unplugged = virtio_pci_device_unplugged;
 }
 
 static const TypeInfo virtio_pci_bus_info = {