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.
