accel: Rename HVF 'struct hvf_vcpu_state' -> AccelCPUState

We want all accelerators to share the same opaque pointer in
CPUState.

Rename the 'hvf_vcpu_state' structure as 'AccelCPUState'.

Use the generic 'accel' field of CPUState instead of 'hvf'.

Replace g_malloc0() by g_new0() for readability.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230624174121.11508-17-philmd@linaro.org>