IOMainPort() is the macOS 12 rename of IOMasterPort(); older SDKs declare only
the latter, so qemu 11.1.0 fails on macOS 11 and earlier.

The signatures are identical, so aliasing suffices.  The guard tests
MAC_OS_X_VERSION_MIN_REQUIRED rather than MAX_ALLOWED: a binary deployed to
macOS 11 must not call a macOS 12 symbol even when built against a newer SDK.

See https://trac.macports.org/ticket/72042

--- block/file-posix.c.orig
+++ block/file-posix.c
@@ -58,6 +58,13 @@
 #include <IOKit/storage/IOCDMedia.h>
 //#include <IOKit/storage/IOCDTypes.h>
 #include <IOKit/storage/IODVDMedia.h>
+
+/* IOMainPort is the macOS 12 rename of IOMasterPort; older SDKs (e.g. 10.15)
+ * declare only the latter.  Same signature, so alias it. */
+#if !defined(MAC_OS_VERSION_12_0) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_VERSION_12_0
+#define IOMainPort IOMasterPort
+#endif
 #include <CoreFoundation/CoreFoundation.h>
 #endif