Step to reproduce: 

I boot from volume like this:

$ nova boot --flavor 2 --key-name sileht --nic net-id=d4021bdd-ac52-44cb-8943-1587c6fe9081 --block-device source=image,id=6f797908-b006-4036-9670-cc3c12c65d56,dest=volume,size=2,shutdown=PRESERVE,bootindex=0 NAME

And then

$ nova shelve 76f4c00d-707a-4426-baba-13217250cec0

Shelving looks ok, now I unshelve:

$ nova unshelve 76f4c00d-707a-4426-baba-13217250cec0

The task state stuck at 'unshelving'

$ nova list
+--------------------------------------+------+-------------------+------------+-------------+---------------------------------+
| ID                                   | Name | Status            | Task State | Power State | Networks                        |
+--------------------------------------+------+-------------------+------------+-------------+---------------------------------+
| 76f4c00d-707a-4426-baba-13217250cec0 | NAME | SHELVED_OFFLOADED | unshelving | Shutdown    | admpriv=192.168.0.5, 172.24.4.4 |
+--------------------------------------+------+-------------------+------------+-------------+---------------------------------+

nova-conductor logs:

2014-04-02 09:51:15.669 ERROR oslo.messaging.rpc.dispatcher [-] Exception during message handling: 'shelved_image_id'
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher Traceback (most recent call last):
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher   File "/vagrant/stack-master/oslo.messaging/oslo/messaging/rpc/dispatcher.py", line 133, in _dispatch_and_reply
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher     incoming.message))
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher   File "/vagrant/stack-master/oslo.messaging/oslo/messaging/rpc/dispatcher.py", line 176, in _dispatch
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher     return self._do_dispatch(endpoint, method, ctxt, args)
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher   File "/vagrant/stack-master/oslo.messaging/oslo/messaging/rpc/dispatcher.py", line 122, in _do_dispatch
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher     result = getattr(endpoint, method)(ctxt, **new_args)
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher   File "/vagrant/stack-master/nova/nova/conductor/manager.py", line 833, in unshelve_instance
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher     sys_meta['shelved_image_id'])
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher KeyError: 'shelved_image_id'
2014-04-02 09:51:15.669 TRACE oslo.messaging.rpc.dispatcher 
2014-04-02 09:51:15.669 ERROR oslo.messaging._drivers.common [-] Returning exception 'shelved_image_id' to caller
2014-04-02 09:51:15.669 ERROR oslo.messaging._drivers.common [-] ['Traceback (most recent call last):\n', '  File "/vagrant/stack-master/oslo.messaging/oslo/messaging/rpc/dispatcher.py", line 133, in _dispatch_and_reply\n    incoming.message))\n', '  File "/vagrant/stack-master/oslo.messaging/oslo/messaging/rpc/dispatcher.py", line 176, in _dispatch\n    return self._do_dispatch(endpoint, method, ctxt, args)\n', '  File "/vagrant/stack-master/oslo.messaging/oslo/messaging/rpc/dispatcher.py", line 122, in _do_dispatch\n    result = getattr(endpoint, method)(ctxt, **new_args)\n', '  File "/vagrant/stack-master/nova/nova/conductor/manager.py", line 833, in unshelve_instance\n    sys_meta[\'shelved_image_id\'])\n', "KeyError: 'shelved_image_id'\n"]


Still present on master (commit 5f8395fe7581b5ef68a1d618831773d473abd017).