virtio_iommu: Clear IOMMUPciBus pointer cache when system reset

s->iommu_pcibus_by_bus_num is a IOMMUPciBus pointer cache indexed
by bus number, bus number may not always be a fixed value,
i.e., guest reboot to different kernel which set bus number with
different algorithm.

This could lead to endpoint binding to wrong iommu MR in
virtio_iommu_get_endpoint(), then vfio device setup wrong
mapping from other device.

Remove the memset in virtio_iommu_device_realize() to avoid
redundancy with memset in system reset.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Message-Id: <20240125073706.339369-2-zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 9a45738)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/virtio/virtio-iommu.c

@@ -1264,6 +1264,8 @@ static void virtio_iommu_system_reset(void *opaque)
 
     trace_virtio_iommu_system_reset();
 
+    memset(s->iommu_pcibus_by_bus_num, 0, sizeof(s->iommu_pcibus_by_bus_num));
+
     /*
      * config.bypass is sticky across device reset, but should be restored on
      * system reset
@@ -1302,8 +1304,6 @@ static void virtio_iommu_device_realize(DeviceState *dev, Error **errp)
 
     virtio_init(vdev, VIRTIO_ID_IOMMU, sizeof(struct virtio_iommu_config));
 
-    memset(s->iommu_pcibus_by_bus_num, 0, sizeof(s->iommu_pcibus_by_bus_num));
-
     s->req_vq = virtio_add_queue(vdev, VIOMMU_DEFAULT_QUEUE_SIZE,
                              virtio_iommu_handle_command);
     s->event_vq = virtio_add_queue(vdev, VIOMMU_DEFAULT_QUEUE_SIZE, NULL);