Currently accel_cpu_realize() only performs target-specific
realization. Introduce the cpu_common_[un]realize fields in
the base AccelClass to be able to perform target-agnostic
[un]realization of vCPUs.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20231003123026.99229-6-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>