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.
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.
Do not color qvm-run diagnostic messages, but also avoid ANSI control
sequences in logs. While at it, do not print 'Running ...' message when
--pass-io is used.
