Rename the commands inside the hotplug scripts to match those used by hotplug

Rename the commands inside the hotplug scripts to match those used by hotplug
and udev.  Pass all commands through from xen-backend.agent or
xen-backend.rules rather than second-guessing the individual scripts.  This
should make it easier to tear down devices cleanly, especially the dangling
vifs we are seeing at the moment.

Cope with a missing type node inside the block script, as we don't want to
resurrect a removed node by writing a hotplug error message.

Log failure of iptables command.

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