exec: Access MemoryRegion with MemOp

exec: Access MemoryRegion with MemOp

The memory_region_dispatch_{read|write} operand "unsigned size" is
being converted into a "MemOp op".

Convert interfaces by using no-op size_memop.

After all interfaces are converted, size_memop will be implemented
and the memory_region_dispatch_{read|write} operand "unsigned size"
will be converted into a "MemOp op".

As size_memop is a no-op, this patch does not change any behaviour.

Signed-off-by: Tony Nguyen <tony.nguyen@bt.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <3b042deef0a60dd49ae2320ece92120ba6027f2b.1566466906.git.tony.nguyen@bt.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>