qapi: support handling VNC events

Even though we do not support VNC events, it can be enabled through
custom arguments. This ensures that the code doesn't crash in an
assertion fail due to an unknown event identifier.

Fixes #3528

qapi/qemu-compat.h

@@ -71,6 +71,7 @@ void qmp_rpc_call(CFDictionaryRef args, CFDictionaryRef *ret, Error **err, void
 
 // TODO: make this match with qemu build
 #define CONFIG_SPICE 1
+#define CONFIG_VNC 1
 #define TARGET_I386 1
 
 #define GCC_FMT_ATTR(n, m)