CA-147820: don't run hotplug add if params is missing

This patch checks the return value of "xenstore-read" when it reads
"backend/vbd/domid/devid/params" while processing an "add" event. This
ensures that we won't write garbage to XenStore, which otherwise
confuses everyone.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>