KVM: x86/pmu: Use separate array for defining "PMU MSRs to save"

Move all potential to-be-saved PMU MSRs into a separate array so that a
future patch can easily omit all PMU MSRs from the list when the PMU is
disabled.

No functional change intended.

Link: https://lore.kernel.org/r/20230124234905.3774678-4-seanjc@google.com


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