virtio-serial-bus: Unset hotplug handler when unrealize

Virtio serial device controls the lifetime of virtio-serial-bus and
virtio-serial-bus links back to the device via its hotplug-handler
property. This extra ref-count prevents the device from getting
finalized, leaving the VirtIODevice memory listener registered and
leading to use-after-free later on.

This patch addresses the same issue as Fam Zheng's
"virtio-scsi: Unset hotplug handler when unrealize"
only for a different virtio device.

Cc: qemu-stable@nongnu.org
Signed-off-by: NLadi Prosek <lprosek@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>

hw/char/virtio-serial-bus.c

@@ -1121,6 +1121,9 @@ static void virtio_serial_device_unrealize(DeviceState *dev, Error **errp)
         timer_free(vser->post_load->timer);
         g_free(vser->post_load);
     }
+
+    qbus_set_hotplug_handler(BUS(&vser->bus), NULL, errp);
+
     virtio_cleanup(vdev);
 }