accel: Rename WHPX 'struct whpx_vcpu' -> AccelCPUState

We want all accelerators to share the same opaque pointer in
CPUState. Rename WHPX 'whpx_vcpu' as 'AccelCPUState'; use
the typedef.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230624174121.11508-14-philmd@linaro.org>