lxc: only write XML once for lxc controller

lxc: only write XML once for lxc controller

Currently when launching the LXC controller we first write out
the plain, inactive XML configuration, then launch the controller,
then replace the file with the live status XML configuration.
By good fortune this hasn't caused any problems other than some
misleading error messages during failure scenarios.

This simplifies the code so it only writes out the XML once and
always writes the live status XML. To do this we need to handshake
with the child process, to make execution pause just before exec()
so we can write the XML status with the child PID present.