KVM: x86/pmu: Don't tell userspace to save MSRs for non-existent fixed PMCs

Limit the set of MSRs for fixed PMU counters based on the number of fixed
counters actually supported by the host so that userspace doesn't waste
time saving and restoring dummy values.

Signed-off-by: Like Xu <likexu@tencent.com>
[sean: split for !enable_pmu logic, drop min(), write changelog]
Link: https://lore.kernel.org/r/20230124234905.3774678-6-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>