tstest/tailmac: add headless mode for automated VM testing

Add a --headless flag to the Host.app Run subcommand for running
macOS VMs without a GUI, enabling use from test frameworks.

Key changes:

  - HostCli.swift: When --headless is set, run the VM via VMController
    + RunLoop.main.run() instead of NSApplicationMain. Using the
    RunLoop (not dispatchMain) is required because VZ framework
    callbacks depend on RunLoop sources.

  - VMController.swift: Add headless parameter to createVirtualMachine
    that configures a single socket-based NIC (no NAT NIC). This
    matches the NIC configuration used when creating/saving VMs, so
    saved state restoration works correctly. A NIC count mismatch
    causes VZ to silently fail to execute guest code.

  - TailMacConfigHelper.swift: Clean up socket network device logging.

  - Config.swift: Move VM storage from ~/VM.bundle to
    ~/.cache/tailscale/vmtest/macos/.

  - TailMac.swift: Fix dispatchMain→RunLoop.main.run() in the create
    command (same VZ RunLoop requirement).

Updates #13038

Change-Id: Iea51c043aa92e8fc6257139b9f0e2e7677072fa2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/tailmac/Swift/Host/HostCli.swift

@@ -20,12 +20,31 @@ extension HostCli {
     struct Run: ParsableCommand {
         @Option var id: String
         @Option var share: String?
+        @Flag(help: "Run without GUI (for automated testing)") var headless: Bool = false
 
         mutating func run() {
             config = Config(id)
             config.sharedDir = share
             print("Running vm with identifier \(id) and sharedDir \(share ?? "<none>")")
-            _ = NSApplicationMain(CommandLine.argc, CommandLine.unsafeArgv)
+
+            if headless {
+                DispatchQueue.main.async {
+                    let controller = VMController()
+                    controller.createVirtualMachine(headless: true)
+
+                    let fileManager = FileManager.default
+                    if fileManager.fileExists(atPath: config.saveFileURL.path) {
+                        print("Restoring virtual machine state from \(config.saveFileURL)")
+                        controller.restoreVirtualMachine()
+                    } else {
+                        print("Starting virtual machine")
+                        controller.startVirtualMachine()
+                    }
+                }
+                RunLoop.main.run()
+            } else {
+                _ = NSApplicationMain(CommandLine.argc, CommandLine.unsafeArgv)
+            }
         }
     }
 }

tstest/tailmac/Swift/Host/VMController.swift

@@ -81,7 +81,7 @@ class VMController: NSObject, VZVirtualMachineDelegate {
         return macPlatform
     }
 
-    func createVirtualMachine() {
+    func createVirtualMachine(headless: Bool = false) {
         let virtualMachineConfiguration = VZVirtualMachineConfiguration()
 
         virtualMachineConfiguration.platform = createMacPlaform()
@@ -90,7 +90,15 @@ class VMController: NSObject, VZVirtualMachineDelegate {
         virtualMachineConfiguration.memorySize = helper.computeMemorySize()
         virtualMachineConfiguration.graphicsDevices = [helper.createGraphicsDeviceConfiguration()]
         virtualMachineConfiguration.storageDevices = [helper.createBlockDeviceConfiguration()]
-        virtualMachineConfiguration.networkDevices = [helper.createNetworkDeviceConfiguration(), helper.createSocketNetworkDeviceConfiguration()]
+        if headless {
+            // In headless mode, use only the socket-based NIC. This matches
+            // the single-NIC configuration used when creating the base VM.
+            // Using a different NIC count would make saved state restoration
+            // fail silently.
+            virtualMachineConfiguration.networkDevices = [helper.createSocketNetworkDeviceConfiguration()]
+        } else {
+            virtualMachineConfiguration.networkDevices = [helper.createNetworkDeviceConfiguration(), helper.createSocketNetworkDeviceConfiguration()]
+        }
         virtualMachineConfiguration.pointingDevices = [helper.createPointingDeviceConfiguration()]
         virtualMachineConfiguration.keyboards = [helper.createKeyboardConfiguration()]
         virtualMachineConfiguration.socketDevices = [helper.createSocketDeviceConfiguration()]