labs: unregister clone VM disk, snapshots

Exporting via cloned VMs leaves the exported disks and snapshots
registered with VirtualBox (whether or not we pass a "--register" flag
to clonevm). Moving the exported VMs away or trying to export again to
the same location makes VirtualBox complain.

If we register the cloned VM upon creation and then unregister it
afterwards, we get at least rid of the snapshots and have only the
base disk left to unregister manually. This rather complicated method
is implemented here.

The export VMs have "-e" attached to their names, making them easier to
unregister here. They are also easy to recognize when imported into a
VirtualBox that already has node VMs using the original names.

Change-Id: I4532fd3152e705c3b35265359d7e6b0d5facd55e
Merge "labs: unregister clone VM disk, snapshots"