Management class for Storage-related functions (attach, detach, etc).
Management class for Volume-related tasks
Attach volume storage to VM instance.
Detach any external nova/cinder volumes and purge the SRs.
Detach volume storage to VM instance.
Find any volumes with their connection severed.
Certain VM operations (e.g. VM.start, VM.reboot, etc.) will not work when a VBD is present that points to a non-working volume. To work around this, we scan for non-working volumes and detach them before retrying a failed operation.