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>

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