hw/arm/virt: Update comment about Multiprocessor Affinity Register

hw/arm/virt: Update comment about Multiprocessor Affinity Register

Support on ARMv7 has been dropped in commit 82bf7ae84ce
("target/arm: Remove KVM support for 32-bit Arm hosts").
Update the comment in virt_cpu_mp_affinity() to avoid
mentioning it.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250429153907.31866-2-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>