accel/whpx: Expose whpx_enabled() to common code

accel/whpx: Expose whpx_enabled() to common code

Currently whpx_enabled() is restricted to target-specific code.
By defining CONFIG_WHPX_IS_POSSIBLE we allow its use anywhere.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250703173248.44995-26-philmd@linaro.org>