KVM: x86/mmu: Revert "Revert "KVM: MMU: show mmu_valid_gen in shadow page related tracepoints""

Now that the fast invalidate mechanism has been reintroduced, restore
tracing of the generation number in shadow page tracepoints.

This reverts commit b59c4830.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>