live migration does not clean up at target node if a failure occurs during post migration

Issue

• Live migration fails during the post processing on the source (i.e. failure to disconnect volumes)
• This lead to the instance being shutdown on the source node and left in a migrating task state.
• Additional a copy of the instance on the target node will be left running although not usable because neutron networking has not yet been switch to target and nova stills records the instance as being on the source node.