VM diagnostics: introduce Diagnostics model object

There is no formal definition for the VM diagnostics. This patch
introduces a model object that will be used by the virt drivers
to return instance diagnostics.

The model will have a version. Each update to the model will require
a version change.

Part of the blueprint v3-diagnostics

Change-Id: I5527bc9904417489ec9bde844c9ff24cb02233fd
Merge "VM diagnostics: introduce Diagnostics model object"