VM diagnostics (v3 API only)

There is no formal definition for the VM diagnostics. For the
V2 API the diagnostics will be returned as they are today.

This will continue to use the driver function get_diagnostics.

This change will be implemented by a new virt driver method called
get_instance_diagnostics.

This will support backward compatibility with the existing API's.

Part of the blueprint v3-diagnostics

This patch set is for the API's. Virt driver support in additional
patches.

DocImpact

Change-Id: Id722f38fb134b322413d2b5c3459c8fc0af7bd36
Merge "VM diagnostics (v3 API only)"