In core-admin matching collections are real dicts, so clone this API
behaviour here too. Specific changes:
- iteration yields keys, not values
- implement values and items methods
Additionally fix keys method, it was broken on python2 (list have no
copy method).
Use newly introduced payload_stream= argument to qubesd_call to pass
data directly from some file-like object - without loading it all into
memory.
QubesOS/qubes-issues#853
