hw/arm/virt: Only describe cpu topology since virt-6.2

hw/arm/virt: Only describe cpu topology since virt-6.2

On existing older machine types, without cpu topology described
in ACPI or DT, the guest will populate one by default. With the
topology described, it will read the information and set up its
topology as instructed, but that may not be the same as what was
getting used by default. It's possible that an user application
has a dependency on the default topology and if the default one
gets changed it will probably behave differently.

Based on above consideration we'd better only describe topology
information to the guest on 6.2 and later machine types.

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20211020142125.7516-2-wangyanan55@huawei.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>