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/Common/TailMacConfigHelper.swift

@@ -83,7 +83,7 @@ struct TailMacConfigHelper {
         // Outbound network packets
         let serverSocket = config.serverSocket
 
-        // Inbound network packets
+        // Inbound network packets — bind a client socket so the server can reply.
         let clientSockId = config.vmID
         let clientSocket = "/tmp/qemu-dgram-\(clientSockId).sock"
 
@@ -118,7 +118,7 @@ struct TailMacConfigHelper {
                                         socklen_t(MemoryLayout<sockaddr_un>.size))
 
         if connectRes == -1 {
-            print("Error binding virtual network server socket - \(String(cString: strerror(errno)))")
+            print("Error connecting to server socket \(serverSocket) - \(String(cString: strerror(errno)))")
             return networkDevice
         }
 
@@ -127,7 +127,6 @@ struct TailMacConfigHelper {
         print("Connected to server at \(serverSocket)")
         print("Socket fd is \(socket)")
 
-
         let handle = FileHandle(fileDescriptor: socket)
         let device = VZFileHandleNetworkDeviceAttachment(fileHandle: handle)
         networkDevice.attachment = device