Thanks Tristan, was this on your rdo kilo env? This looks like you've hit OSSA 2016-001 / bug#1524274, which package version are you using? Anything prior to 2015.1.3 will hit this, I'm following up with the RDO team to ensure this version is available now. 

Looking into the unshelve case we appear protected by a check within nova/virt/images.fetch_to_raw that correctly detects that a backing_file is now listed and blocks any attempt at unshelving the instance :

2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [req-e3d11cbb-3a55-4eaa-ac99-592c751209ea 4e15d42d6f2d40c781b3097315a835af 37e7ea4bcdfc471191ea1267e7e6b4e0 - - -] [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9] Instance failed to spawn
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9] Traceback (most recent call last):
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 4316, in _unshelve_instance
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     block_device_info=block_device_info)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 2516, in spawn
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     admin_pass=admin_password)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 2940, in _create_image
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     instance, size, fallback_from_host)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 6409, in _try_fetch_image_cache
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     size=size)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/imagebackend.py", line 240, in cache
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     *args, **kwargs)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/imagebackend.py", line 472, in create_image
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     prepare_template(target=base, max_size=size, *args, **kwargs)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/oslo_concurrency/lockutils.py", line 254, in inner
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     return f(*args, **kwargs)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/imagebackend.py", line 230, in fetch_func_sync
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     fetch_func(target=target, *args, **kwargs)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/utils.py", line 406, in fetch_image
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     max_size=max_size)
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]   File "/usr/lib/python2.7/site-packages/nova/virt/images.py", line 126, in fetch_to_raw
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9]     {'fmt': fmt, 'backing_file': backing_file}))
2016-02-26 06:40:17.248 3903 ERROR nova.compute.manager [instance: 8dcd132e-2083-4987-8197-fd56d6ce38e9] ImageUnacceptable: Image 2cb1fb9e-f628-4d3f-8de8-09070c5de7fa is unacceptable: fmt=qcow2 backed by: /dev/sda3


So it looks like we still only have the live and cold migration cases to fix at the moment.