add lxc.console.logpath

logpath has been supported through lxc-start command line, but not
through the API.  Since the lxc.console is now required to be a device,
support lxc.console.logfile to be a simple file to which console output
will be logged.

clear_config_item is not supported, as it isn't for lxc.console, bc
you can do 'lxc.console.logfile =' to clear it.

(This patch is for stable-1.0)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>