Fixes bug 989306 If nova-compute is killed, or the compute domU is shutdown uncleanly, instance VDIs can remain attached to the compute domU. This complicates cleaning up unused VDIs, but can also result in reaching a limit on the number of VBDs for the compute domU, preventing new instances from being built. Change-Id: Ie56e0561aa02178d6718b74a87bbb591fa869007