accel: Rename NVMM 'struct qemu_vcpu' -> AccelCPUState

We want all accelerators to share the same opaque pointer in
CPUState. Rename NVMM 'qemu_vcpu' as 'AccelCPUState'; directly
use the typedef, remove unnecessary casts.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230624174121.11508-11-philmd@linaro.org>