Nova API > 2.87 breaks fetching cpu_info from hypervisor

[samuelallan72]

Please see discussion at openstack-exporter/openstack-exporter#268.

The root of the error is at

gophercloud/openstack/compute/v2/extensions/hypervisors/results.go

Lines 166 to 176 in 51f2469

	switch t := s.CPUInfo.(type) {
	case string:
	tmpb = []byte(t)
	case map[string]interface{}:
	tmpb, err = json.Marshal(t)
	if err != nil {
	return err
	}
	default:
	return fmt.Errorf("CPUInfo has unexpected type: %T", t)
	}

Here, with Nova API > 2.87, CPUInfo is nil, causing the error.