cpus: Register VMState per user / system emulation

Simplify cpu-target.c by extracting mixed vmstate code
into the cpu_vmstate_register() / cpu_vmstate_unregister()
helpers, implemented in cpu-user.c and cpu-system.c.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250123234415.59850-20-philmd@linaro.org>
4 files changed