vmnet_enable_isolation_key is macOS 11, so older SDKs do not declare it.

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

--- net/vmnet-bridged.m.orig
+++ net/vmnet-bridged.m
@@ -88,6 +88,15 @@
         return false;
     }
 
+#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-bridged.isolated feature is "
+                   "unavailable: outdated vmnet.framework API");
+        return false;
+    }
+#endif
     return true;
 }
 
@@ -106,10 +115,12 @@
                               vmnet_shared_interface_name_key,
                               options->ifname);
 
+#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;
 }