vmnet.framework's VMNET_SHARING_SERVICE_BUSY is macOS 11, so older SDKs do not
declare it.

This restores the guard qemu carried until commit 3bf445fbb199 ("net/vmnet:
Drop ifdef for macOS versions older than 12.0"), which removed pre-12
back-compat wholesale.  The code here is that commit reverted.

--- net/vmnet-common.m.orig
+++ net/vmnet-common.m
@@ -48,8 +48,11 @@
         return "buffers exhausted in kernel";
     case VMNET_TOO_MANY_PACKETS:
         return "packet count exceeds limit";
+#if defined(MAC_OS_VERSION_11_0) && \
+    MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_VERSION_11_0
     case VMNET_SHARING_SERVICE_BUSY:
         return "conflict, sharing service is in use";
+#endif
     default:
         return "unknown vmnet error";
     }