Migrate of instance with no local storage fails with exception

Since migration operates by means of a resize, the code attempts to resize the instance from 0 to 0 GB.

2011-07-13 01:14:36,323 DEBUG nova.virt.xenapi.vmops [-] Resizing VDI 90e2ee60-cbbe-429e-abf9-e0f2f813462d for instance instance-0000000a. Expanding to 0 GB from (pid=14629) resize_instance /home/johannes/openstack/nova/trunk/nova/virt/xenapi/vmops.py:577

This results in an exception:

2011-07-13 01:14:36,607 ERROR nova.exception [-] Uncaught exception
(nova.exception): TRACE: Traceback (most recent call last):
(nova.exception): TRACE: File "/home/johannes/openstack/nova/trunk/nova/exception.py", line 87, in _wrap
(nova.exception): TRACE: return f(*args, **kw)
(nova.exception): TRACE: File "/home/johannes/openstack/nova/trunk/nova/compute/manager.py", line 107, in decorated_function
(nova.exception): TRACE: function(self, context, instance_id, *args, **kwargs)
(nova.exception): TRACE: File "/home/johannes/openstack/nova/trunk/nova/compute/manager.py", line 828, in finish_resize
(nova.exception): TRACE: self.driver.finish_resize(instance, disk_info, network_info)
(nova.exception): TRACE: File "/home/johannes/openstack/nova/trunk/nova/virt/xenapi_conn.py", line 207, in finish_resize
(nova.exception): TRACE: self._vmops.finish_resize(instance, disk_info, network_info)
(nova.exception): TRACE: File "/home/johannes/openstack/nova/trunk/nova/virt/xenapi/vmops.py", line 119, in finish_resize
(nova.exception): TRACE: self.resize_instance(instance, vdi_uuid)
(nova.exception): TRACE: File "/home/johannes/openstack/nova/trunk/nova/virt/xenapi/vmops.py", line 579, in resize_instance
(nova.exception): TRACE: self._session.call_xenapi('VDI.resize_online', vdi_ref, new_disk_size)
(nova.exception): TRACE: File "/home/johannes/openstack/nova/trunk/nova/virt/xenapi_conn.py", line 372, in call_xenapi
(nova.exception): TRACE: return tpool.execute(f, *args)
(nova.exception): TRACE: File "/usr/local/lib/python2.6/dist-packages/eventlet/tpool.py", line 76, in tworker
(nova.exception): TRACE: rv = meth(*args,**kwargs)
(nova.exception): TRACE: File "/usr/local/lib/python2.6/dist-packages/XenAPI.py", line 229, in __call__
(nova.exception): TRACE: return self.__send(self.__name, args)
(nova.exception): TRACE: File "/usr/local/lib/python2.6/dist-packages/XenAPI.py", line 133, in xenapi_request
(nova.exception): TRACE: result = _parse_result(getattr(self, methodname)(*full_params))
(nova.exception): TRACE: File "/usr/local/lib/python2.6/dist-packages/XenAPI.py", line 203, in _parse_result
(nova.exception): TRACE: raise Failure(result['ErrorDescription'])
(nova.exception): TRACE: Failure: ['SR_BACKEND_FAILURE_79', '', 'VDI Invalid size [opterr=shrinking not allowed]', '']