accel: Destroy HAX vCPU threads once done

accel: Destroy HAX vCPU threads once done

When the vCPU thread finished its processing, destroy
it and signal its destruction to generic vCPU management
layer.

Add a sanity check for the vCPU accelerator context.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230624174121.11508-6-philmd@linaro.org>