When qrexec_client cannot write to its stdout, this doesn't necessary mean that
there is no data in opposite direction.
Simple example is RPC service: when process in destination VM closes its stdin,
it can still send some data to triggering VM.
If we do not wait and exit imemdiately, qrexec_daemon will decrease
the children count and continue spawning processes, while e.g.
qfile-daemon still waits for kdialog - so dom0 will be DoSed by
multiple processes.
