This allows to perform actions on objects (VM, storage etc), without
listing them. This is useful when calling VM have minimal permissions
and only selected actions are allowed.
This means that app.domains['some-name'] will not raise KeyError, even
when domain do not exists. But performing actual action (like
vm.start()) will fail in that case.
Add qubesd_call(..., payload_stream=...) argument to allow streaming
payload directly from some file/process stdout. This is mainly (only?)
useful for admin.vm.volume.Import, where disk volume raw data is passed
to the service.
