Log errors from hotplug scripts and continue

It's better to continue with a cleanup than leave a VM in a bad state
(especially given that we aren't currently relying on the hotplug scripts
to do anything particularly significant)

In the case of an HVM CDROM which is ejected, the "type" key will be
set to "" which causes /etc/xen/scripts/block to fail.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
If we're running hotplug scripts manually, then wait for the backend to close, not for the hotplug scripts to run (!)

Fixes #16

Signed-off-by: David Scott <dave.scott@eu.citrix.com>