Add handling for reboots of LXC containers

Add handling for reboots of LXC containers

The reboot() syscall is allowed by new kernels for LXC containers.
The LXC controller can detect whether a reboot was requested
(instead of a normal shutdown) by looking at the "init" process
exit status. If a reboot was triggered, the exit status will
record SIGHUP as the kill reason.

The LXC controller has cleared all its capabilities, and the
veth network devices will no longer exist at this time. Thus
it cannot restart the container init process itself. Instead
it emits an event which is picked up by the LXC driver in
libvirtd. This will then re-create the container, using the
same configuration as it was previously running with (ie it
will not activate 'newDef').

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>