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>
CA-113235: Don't call init.d scripts directly

Some daemons are launched invoking /etc/init.d scripts directly. This
could be a problem in environment with upstart which wants to know the
status of these services. For this reason it's better to use the {service}
command to notify upstart.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
XOP-435: Allow clearing of DNS entries.
Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Merge pull request #1497 from euanh/CA-113235

CA-113235: Don't call init.d scripts directly
Merge pull request #1495 from simonjbeaumont/cp-5970

CP-5970: Move xenops' cancellable watch paths
Merge pull request #1468 from siddharthv/clear-dns

XOP-435: Allow clearing of DNS entries.