[libvirt] Handle reboots in LXC driver

This is a series which makes it possible to properly handle reboots in the LXC driver. The lxc_controller can detect if the container asked for a reboot by looking at the exit status of the init process & checking for SIGHUP as the termination signal. The fun is that the lxc controller cannot actually restart the container itself though. It lacks the permissions todo so (it dropped all capabilities), and it is not able to configure the veth devices either. So we introduce an RPC protocol with an event notification on shutdown, to tell libvirtd that a restart is needed This applies on top of https://www.redhat.com/archives/libvir-list/2012-July/msg01309.html