CP-5970: Move xenops' cancellable watch paths

Having these paths created were interferring with devices since they were being
recreated once the device had beeen destroyed in the case of VBDs. Now we'll
keep track of the cancellable watch paths in a separate subtree of XenStore and
also tie them to their Xenops task IDs to aid debugging. Also, the semantics of
how to cancel the task have been added (removing the key).

Old path: /local/domain/0/backend/vbd/<domid>/<devid>/tools/xenops/cancel

New path: /xenops/task/<task-id>/remove-to-cancel

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>