tstest/tailmac: add support for mounting host directories in the guest (#13957)

updates tailscale/corp#24197

tailmac run now supports the --share option which will allow you
to specify a directory on the host which can be mounted in the guest
using  mount_virtiofs vmshare <path>.

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

tstest/tailmac/Swift/Common/Config.swift

@@ -14,6 +14,7 @@ class Config: Codable {
     var mac = "52:cc:cc:cc:cc:01"
     var ethermac = "52:cc:cc:cc:ce:01"
     var port: UInt32 = 51009
+    var sharedDir: String?
 
     // The virtual machines ID.  Also double as the directory name under which
     // we will store configuration, block device, etc.

tstest/tailmac/Swift/Common/TailMacConfigHelper.swift

@@ -141,5 +141,18 @@ struct TailMacConfigHelper {
     func createKeyboardConfiguration() -> VZKeyboardConfiguration {
         return VZMacKeyboardConfiguration()
     }
+
+    func createDirectoryShareConfiguration(tag: String) -> VZDirectorySharingDeviceConfiguration? {
+        guard let dir = config.sharedDir else { return nil }
+
+        let sharedDir = VZSharedDirectory(url: URL(fileURLWithPath: dir), readOnly: false)
+        let share = VZSingleDirectoryShare(directory: sharedDir)
+
+        // Create the VZVirtioFileSystemDeviceConfiguration and assign it a unique tag.
+        let sharingConfiguration = VZVirtioFileSystemDeviceConfiguration(tag: tag)
+        sharingConfiguration.share = share
+
+        return sharingConfiguration
+    }
 }