qemu: use systemd's TerminateMachine to kill all processes

qemu: use systemd's TerminateMachine to kill all processes

If we don't properly clean up all processes in the
machine-<vmname>.scope systemd won't remove the cgroup and subsequent vm
starts fail with

  'CreateMachine: File exists'

Additional processes can e.g. be added via

  echo $PID > /sys/fs/cgroup/systemd/machine.slice/machine-${VMNAME}.scope/tasks

but there are other cases like

  http://bugs.debian.org/761521

Invoke TerminateMachine to be on the safe side since systemd tracks the
cgroup anyway. This is a noop if all processes have terminated already.