tstest/tailmac: add customized macOS virtualization tooling (#13146)

updates tailcale/corp#22371

Adds custom macOS vm tooling.  See the README for
the general gist, but this will spin up VMs with unixgram
capable network interfaces listening to a named socket,
and with a virtio socket device for host-guest communication.

We can add other devices like consoles, serial, etc as needed.

The whole things is buildable with a single make command, and
everything is controllable via the command line using the TailMac
utility.

This should all be generally functional but takes a few shortcuts
with error handling and the like.  The virtio socket device support
has not been tested and may require some refinement.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

tstest/tailmac/Swift/Host/HostCli.swift

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+import Cocoa
+import Foundation
+import Virtualization
+import ArgumentParser
+
+@main
+struct HostCli: ParsableCommand {
+    static var configuration = CommandConfiguration(
+        abstract: "A utility for running virtual machines",
+        subcommands: [Run.self],
+        defaultSubcommand: Run.self)
+}
+
+var config: Config = Config()
+
+extension HostCli {
+    struct Run: ParsableCommand {
+        @Option var id: String
+
+        mutating func run() {
+            print("Running vm with identifier \(id)")
+            config = Config(id)
+            _ = NSApplicationMain(CommandLine.argc, CommandLine.unsafeArgv)
+        }
+    }
+}
+