Unrescue causes 'Device in use' when trying to unplug VBD devices

I get the following exceptions when trying to unrescue an instance in which I have mounted the original image:

(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/nova/compute/manager.py", line 115, in decorated_function
(nova.exception): TRACE: function(self, context, instance_id, *args, **kwargs)
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/nova/compute/manager.py", line 833, in unrescue_instance
(nova.exception): TRACE: self.driver.unrescue(instance_ref, None, network_info)
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/nova/virt/xenapi_conn.py", line 251, in unrescue
(nova.exception): TRACE: self._vmops.unrescue(instance, _callback)
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/nova/virt/xenapi/vmops.py", line 1006, in unrescue
(nova.exception): TRACE: self._destroy_rescue_instance(rescue_vm_ref)
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/nova/virt/xenapi/vmops.py", line 888, in _destroy_rescue_instance
(nova.exception): TRACE: self._destroy_rescue_vbds(rescue_vm_ref)
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/nova/virt/xenapi/vmops.py", line 830, in _destroy_rescue_vbds
(nova.exception): TRACE: VMHelper.unplug_vbd(self._session, vbd_ref)
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/nova/virt/xenapi/vm_utils.py", line 275, in unplug_vbd
(nova.exception): TRACE: raise StorageError(_('Unable to unplug VBD %s') % vbd_ref)
(nova.exception): TRACE: StorageError: Unable to unplug VBD OpaqueRef:273ad420-f138-0818-8ad5-61bcec1a42c0

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There really is no need to attempt to hot unplug VBD devices. We should simply shutdown the rescue instance, wait for shutdown to complete, and then destroy the VDI.