qom: aggressively optimize qom casting

This patch adds a small typename cache to ObjectClass.  This allows
caching positive casts within each ObjectClass.  Benchmarking a
PPC workload provided by Aurelien, this patch eliminates every
single g_hash_table_lookup() happening during the benchmark (which
was about 2 million per-second).

With this patch applied, I get exactly the same performance (within
the margin of error) as with --disable-qom-cast-debug.

N.B. it's safe to cache typenames only from the _assert() macros
because they are always called with string literals.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

include/qom/object.h

@@ -344,6 +344,8 @@ typedef void (ObjectUnparent)(Object *obj);
  */
 typedef void (ObjectFree)(void *obj);
 
+#define OBJECT_CLASS_CAST_CACHE 4
+
 /**
  * ObjectClass:
  *
@@ -356,6 +358,8 @@ struct ObjectClass
     Type type;
     GSList *interfaces;
 
+    const char *cast_cache[OBJECT_CLASS_CAST_CACHE];
+
     ObjectUnparent *unparent;
 };