system/hvf: Expose hvf_enabled() to common code

system/hvf: Expose hvf_enabled() to common code

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

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250403235821.9909-28-philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>