KVM: selftests: Verify LBRs are disabled if vPMU is disabled

Verify that disabling the guest's vPMU via CPUID also disables LBRs.
KVM has had at least one bug where LBRs would remain enabled even though
the intent was to disable everything PMU related.

Link: https://lore.kernel.org/r/20230311004618.920745-22-seanjc@google.com


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