Introduce a structure to hold handler specific to sysemu.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210517105140.1062037-15-f4bug@amsat.org>
[rth: Squash "restrict hw/core/sysemu-cpu-ops.h" patch]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>