katautils: Add Darwin stub for the netNS API

And move the current implementation into a Linux only file.

Signed-off-by: Samuel Ortiz <s.ortiz@apple.com>

src/runtime/pkg/katautils/network_darwin.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2022 Apple Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package katautils
+
+import (
+	vc "github.com/kata-containers/kata-containers/src/runtime/virtcontainers"
+)
+
+func EnterNetNS(networkID string, cb func() error) error {
+	return nil
+}
+
+func SetupNetworkNamespace(config *vc.NetworkConfig) error {
+	return nil
+}
+
+func cleanupNetNS(netNSPath string) error {
+	return nil
+}