Change the way that reboot-timeouts are handled. Rather than refreshing the

Change the way that reboot-timeouts are handled.  Rather than refreshing the
list of domains each time XendDomainInfo.refresh is called, and spawning
threads for each shutdown still in progress, we watch the control/shutdown
node and spawn just the one thread immediately.  When the daemon starts (say
after a crash) all domains are checked for entries in the store indicating a
reboot in progress.

This avoids a large cost on the store and in threads when the list of domains
is refreshed.  This happens every time the server is queried, so the cost can
be quite high.  It's especially a high cost when using xm shutdown -a -w --
this operation should perform a lot better now.

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