Remove the reason code from the destroy action -- the reason is only of value

Remove the reason code from the destroy action -- the reason is only of value
on a graceful shutdown.  Clarify the semantic difference between poweroff and
halt (we use halt to mean 'shutdown and stop' and poweroff to mean 'shutdown
and do whatever the configured behaviour is').

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