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

===

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

/reviewed-on nexedi/slapos!585

software/kvm/buildout.hash.cfg

@@ -59,7 +59,7 @@ md5sum = 2036bf145f472f62ef8dee5e729328fd
 
 [template-kvm-run]
 filename = template/template-kvm-run.in
-md5sum = c8ca875bf246997137552538ab9d8b1a
+md5sum = 2a49e6065a3f46f871318ba88f0cd235
 
 [template-kvm-controller]
 filename = template/kvm-controller-run.in

software/kvm/template/template-kvm-run.in

@@ -302,7 +302,7 @@ kvm_argument_list = [qemu_path,
                                                  disk_aio, additional_disk_options),
   '-vnc', '%s:1,ipv4,password' % listen_ip,
   '-boot', 'order=cd,menu=on',
-  '-qmp', 'unix:%s,server' % socket_path,
+  '-qmp', 'unix:%s,server,nowait' % socket_path,
   '-pidfile', pid_file_path, '-msg', 'timestamp=on',
   '-D', logfile,
 ]