CA-137467: Move get_private_path into shared location

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CA-137467: Move cancel paths into private XenStore tree

Move all of the xenopsd paths out of the paths used by the XenBus protocol
(/local/domain/<backend-domid>/<device-kind>/<frontend-domid>/...) and into the
already-existing private subtree it maintains.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1822 from simonjbeaumont/xenops-paths-cr3

[xs64bit] Move Xenops related data into private XenStore subtree