KVM: selftests: Print (previous) last_page on dirty page value mismatch

Print out the last dirty pages from the current and previous iteration on
verification failures.  In many cases, bugs (especially test bugs) occur
on the edges, i.e. on or near the last pages, and being able to correlate
failures with the last pages can aid in debug.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20250111003004.1235645-14-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/dirty_log_test.c

@@ -566,8 +566,10 @@ static void vm_dirty_log_verify(enum vm_guest_mode mode, unsigned long *bmap)
 				}
 			}
 
-			TEST_FAIL("Dirty page %lu value (%lu) != iteration (%lu)",
-				  page, val, iteration);
+			TEST_FAIL("Dirty page %lu value (%lu) != iteration (%lu) "
+				  "(last = %lu, prev_last = %lu)",
+				  page, val, iteration, dirty_ring_last_page,
+				  dirty_ring_prev_iteration_last_page);
 		} else {
 			nr_clean_pages++;
 			/*
@@ -590,9 +592,10 @@ static void vm_dirty_log_verify(enum vm_guest_mode mode, unsigned long *bmap)
 			 *     value "iteration-1".
 			 */
 			TEST_ASSERT(val <= iteration,
-				    "Clear page %"PRIu64" value %"PRIu64
-				    " incorrect (iteration=%"PRIu64")",
-				    page, val, iteration);
+				    "Clear page %lu value (%lu) > iteration (%lu) "
+				    "(last = %lu, prev_last = %lu)",
+				    page, val, iteration, dirty_ring_last_page,
+				    dirty_ring_prev_iteration_last_page);
 			if (val == iteration) {
 				/*
 				 * This page is _just_ modified; it