libvirt: disconnect volume from host during detach

Issue

• During a volume detach operation, Nova compute attempts to remove the volume from libvirt for the instance before proceeding to remove the storage lun from the underlying compute host.
• If Nova discovers that the volume was not found in the instance's libvirt definition then it ignores that error condition and returns (after issuing a warning message "Ignoring DiskNotFound exception while detaching").