XenAPI: VM diagnostics for v3 API

For V3 API the diagnostics will have the format defined at:
https://wiki.openstack.org/wiki/Nova_VM_Diagnostics

This will support backward compatability with the existing APIs

Part of the blueprint v3-diagnostics

DocImpact

Change-Id: I8377f86a97ca554028e4ca8c1c1af7fd5643cc2e
Merge "XenAPI: VM diagnostics for v3 API"