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.
Don't terminate qvm-run on any SIGCHLD, check if the process we're
waiting for have finished.
Currently the only situation when it's broken is a test (which starts
additional process, whose SIGCHLD may be caught here), but lets do not
assume that much (starting only one process) about environment.
