Fix HVM shutdown when xend is restarted.

Fix HVM shutdown when xend is restarted.

Added a recreate call to ImageHandler, allowing the subclasses of that to
hook into the code that runs when xend restarts.  This allows us in particular
to reregister the watches for HVM shutdown, and read the PID of qemu-dm from
the store.

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