KVM: x86/pmu: Update comments for AMD gp counters

The obsolete comment could more accurately state that AMD platforms
have two base MSR addresses and two different maximum numbers
for gp counters, depending on the X86_FEATURE_PERFCTR_CORE feature.

Signed-off-by: Like Xu <likexu@tencent.com>
Message-Id: <20220518132512.37864-2-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>