vmnet_enable_isolation_key and vmnet_network_identifier_key are macOS 11, so
older SDKs do not declare them.

This restores the guards qemu carried until commit 3bf445fbb199 ("net/vmnet:
Drop ifdef for macOS versions older than 12.0").

--- net/vmnet-shared.c.orig
+++ net/vmnet-shared.c
@@ -20,6 +20,16 @@
 static bool validate_options(const Netdev *netdev, Error **errp)
 {
     const NetdevVmnetSharedOptions *options = &(netdev->u.vmnet_shared);
+
+#if !defined(MAC_OS_VERSION_11_0) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_VERSION_11_0
+    if (options->has_isolated) {
+        error_setg(errp,
+                   "vmnet-shared.isolated feature is "
+                   "unavailable: outdated vmnet.framework API");
+        return false;
+    }
+#endif
 
     if ((options->start_address ||
          options->end_address ||
@@ -66,11 +76,14 @@
                                   options->subnet_mask);
     }
 
+#if defined(MAC_OS_VERSION_11_0) && \
+    MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_VERSION_11_0
     xpc_dictionary_set_bool(
         if_desc,
         vmnet_enable_isolation_key,
         options->isolated
     );
+#endif
 
     return if_desc;
 }