It's already available in config dict, but lets provide uniform API. And
also it's a bit weird to look for usage data in configuration...
QubesOS/qubes-issues#3240
This is to match core-admin API. Logically it could be a property not a
method (as other things like usage or size), but it is already defined
(and used in various places) as a method in core-admin, so lets don't
change the API right now.
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
