libvirt: 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 support backward compatibility with the existing API's.

Part of the blueprint v3-diagnostics

DocImpact

Change-Id: I4128e76622e73a40d25fb50b10b1c1ac710a5cc4
Merge "libvirt: VM diagnostics (v3 API only)"