Description of problem:

when we try to migrate or resize an instance (not a live migration), the instance status is switched to "Error" even if the resize was done successfully.

In the nova log file, we have this : 

~~~
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 142, in _dispatch_and_reply
    executor_callback))
  File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 186, in _dispatch
    executor_callback)
  File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 129, in _do_dispatch
    result = func(ctxt, **new_args)
  File "/usr/lib/python2.7/site-packages/nova/exception.py", line 89, in wrapped
    payload)
  File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 204, in __exit__
    six.reraise(self.type_, self.value, self.tb)
  File "/usr/lib/python2.7/site-packages/nova/exception.py", line 72, in wrapped
    return f(self, context, *args, **kw)
  File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 400, in decorated_function
    return function(self, context, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 378, in decorated_function
    kwargs['instance'], e, sys.exc_info())
  File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 204, in __exit__
    six.reraise(self.type_, self.value, self.tb)
  File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 366, in decorated_function
    return function(self, context, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 3428, in confirm_resize
    do_confirm_resize(context, instance, migration.id)
  File "/usr/lib/python2.7/site-packages/oslo_concurrency/lockutils.py", line 254, in inner
    return f(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 3426, in do_confirm_resize
    migration=migration)
  File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 3453, in _confirm_resize
    network_info)
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 7199, in confirm_migration
    self._cleanup_resize(instance, network_info)
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 1050, in _cleanup_resize
    ignore_errors=True)
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/imagebackend.py", line 885, in remove_snap
    return self.driver.remove_snap(self.rbd_name, name, ignore_errors)
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/storage/rbd_utils.py", line 391, in remove_snap
    with RBDVolumeProxy(self, volume) as vol:
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/storage/rbd_utils.py", line 65, in __init__
    driver._disconnect_from_rados(client, ioctx)
  File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 204, in __exit__
    six.reraise(self.type_, self.value, self.tb)
  File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/storage/rbd_utils.py", line 61, in __init__
    read_only=read_only)
  File "/usr/lib/python2.7/site-packages/rbd.py", line 374, in __init__
    raise make_ex(ret, 'error opening image %s at snapshot %s' % (name, snapshot))
ImageNotFound: error opening image dda93e33-f1a3-450c-bfaf-52cc2747fb10_disk at snapshot None
~~~

Version-Release number of selected component (if applicable):
RHEL OSP 8
openstack-nova-compute-12.0.3-1.el7ost.noarch

How reproducible:
Everytime for Cu 

Steps to Reproduce:

- ceph is configured as cinder backend.
- You have instance booted from rbd volume [boot from volume]
- Resize the instance.

Actual results:
Instance is ending with error state.

Expected results:
It should get completed successfully.

Additional info:

Issue is already reported upstream [1]
Got a confirmation from Cu that issue is fixed after applying the patch [2]

[1] https://bugs.launchpad.net/nova/+bug/1580625
[2] https://git.openstack.org/cgit/openstack/nova/commit/?id=743d5efccaa99e3b4873831a8f43c216a31c7113


I checked the changelog of the recent openstack-nova version till the version which Cu is using but I didn't find the bug fixed in that version.