Add the config file parsing for the on_{poweroff,reboot,crash} options, so that

Add the config file parsing for the on_{poweroff,reboot,crash} options, so that
they actually take effect.  Added behaviour "rename-restart" for debugging
purposes, that renames the domain out of the way, preserving it for debugging,
but starts a new domain too.

Add explicit remove of old domain paths when creating a new domain, to avoid
stale information affecting us (by shutting the domain down, for example).

Signed-off-by: Ewan Mellor <ewan@xensource.com>