KVM: selftests: Tighten checks around prev iter's last dirty page in ring

KVM: selftests: Tighten checks around prev iter's last dirty page in ring

Now that each iteration collects all dirty entries and ensures the guest
*completes* at least one write, tighten the exemptions for the last dirty
page of the previous iteration.  Specifically, the only legal value (other
than the current iteration) is N-1.

Unlike the last page for the current iteration, the in-progress write from
the previous iteration is guaranteed to have completed, otherwise the test
would have hung.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20250111003004.1235645-18-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>