Configure qemu explicitly to not wait for connection on QMP socket

otherwise it will stall VM execution.

This is observed inside qemu VM started by

https://lab.nexedi.com/nexedi/slapos/blob/master/software/erp5testnode/testsuite/deploy-test/software.cfg

In qemu log one can see:

QEMU waiting for connection on: disconnected:unix:/srv/slapgrid/slappart6/var/qmp_socket,server

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Affected version:

root@z600:/srv/slapgrid/slappart6# /opt/slapgrid/7903668a197f3cf8f94375d0739a21e8/parts/kvm/bin/qemu-system-x86_64 --version QEMU emulator version 2.8.0 Copyright (c) 2003-2016 Fabrice Bellard and the QEMU Project developer