Handle NotImplementedError in server_diagnostics v3 api

In libvirt driver, it doesn't implement the function of
get_instance_diagnostics().it returns http 500 error No.
we expect http 501 if the function is not implemented.

Change-Id: I59bfaa27b96284bd56917096e0c3fab0d277ba96
Closes-Bug: 1349680
Merge "Handle NotImplementedError in server_diagnostics v3 api"