Code review comment for lp://qastaging/~danilo/simplestreams/insert-item-factoring

Danilo, thank you for cleanups. Looks mostly good.

The one thing i'm concerned about is changing the signature on GlanceMirror.

I'm not opposed to variable name improvements. but maybe something a bit shorrter.
  flattened_img_data
  target_sstream_item

clearly we're in bike shed territory here.

do you think its better or worse to have hypervisor_mapping at the class level ?
rather than getting it in insert_item and passing it around.