Comment # 2 on bug 933132 from Alexander Graf

The abort() triggers because we're trying to access the COWed original L2 table
entry and the current non-COWed L2 table entry via the same L2 table which can
only hold 1 active, in use entry at a time.

Thanks to this report this was fixed upstream in

  https://lists.gnu.org/archive/html/qemu-block/2015-06/msg00019.html

and I'll cherry pick this into our tree.