accel: Rename HAX 'struct hax_vcpu_state' -> AccelCPUState

accel: Rename HAX 'struct hax_vcpu_state' -> AccelCPUState

We want all accelerators to share the same opaque pointer in
CPUState. Start with the HAX context, renaming its forward
declarated structure 'hax_vcpu_state' as 'AccelCPUState'.
Document the CPUState field. Directly use the typedef.

Remove the amusing but now unnecessary casts in NVMM / WHPX.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230624174121.11508-8-philmd@linaro.org>