So in an attempt to fix change CD functionality we discovered a few other 
potential issues and what Nir suggested was to implement two [somewhat] new 
functions in VDSM: hotplug and hotunplug for CDs similar to how it works for 
normal disks now. Existing changeCD function will be left as is for backwards 
As I found out, engine already calculates iface and index before invoking VDSM 
functions, so we will just pass these along with PDIV to the VDSM.

Suggested flow is, let me quote:

>So the complete change CD flow should be:
>1. get the previous drivespec from vm metadata
>2. prepare new drivespec
>3. add new drivespec to vm metadata
>4. attach a new device to vm
>5. teardown the previous drivespec
>6. remove previous drivespec from vm metadata
>When the vm is stopped, it must do:
>1. get drive spec from vm metadata
>2. teardown drivespec
>During attach, there are interesting races:
>- what happens if vdsm crashes after step 2? who will teardown the volume?
>  maybe you need to add the new drivespec to the metadata first,
>before preparing it.
>- what happens if attach failed? who will remove the new drive from
>the metadata?

Now, what makes hotplugDisk/hotunplugDisk different? From what I understand, 
the flow is same there, so what difference is there as far as VDSM is 
concerned? If none, this means if I more or less copy that code, changing minor 
details and data accordingly for CDs, this should work, shouldn't it?

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