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.
It is supported only from dom0, but it's still useful to have, to save
on simultaneous vchan connections (only waiting for MSG_DATA_EXIT_CODE).
This is especially important for Windows VMs, as qrexec-agent there have
pretty low limit on simultaneous connections (about 20).
Make qvm-run use it.
