sriov: add scan-cycle fallback for verify_irqbalance on managed-IRQ VMs

On v6 SKUs with MANA NICs, all network IRQs use managed mode where the
kernel controls affinity. With 32+ cores distributing IRQs evenly across
cache domains, irqbalance correctly determines no rebalancing is needed
and never prints 'Selecting irq X for rebalancing'. This causes
verify_irqbalance to fail even though irqbalance is working correctly.

Add a fallback assertion: when no active rebalancing is detected, verify
that irqbalance completed at least one scan cycle by checking for
separator lines and 'Interrupt N node_num' messages in its debug output.
This confirms irqbalance ran and scanned the interrupt topology, even if
it decided nothing needed moving.

Tested on:
- RHEL 9.0 / Standard_E32ds_v6 (previously failing): PASSED 2/2
- Ubuntu 22.04 / Standard_D8ds_v5 (regression check): PASSED 1/1

Author: kevinnelson-affirmed

lisa/microsoft/testsuites/network/sriov.py

@@ -772,12 +772,29 @@ def verify_irqbalance(self, environment: Environment, log: Logger) -> None:
         # before waiting for the result, so wait_result() does not time out.
         server_node.tools[Kill].by_name("irqbalance", ignore_not_exist=True)
         result = irqbalance.wait_result(raise_on_timeout=False)
-        assert re.search(
+
+        # Check that irqbalance actively rebalanced an IRQ.
+        selecting_match = re.search(
             "Selecting irq [0-9]+ for rebalancing",
             result.stdout,
-        ), (
-            "irqbalance is not rebalancing irqs" + err_msg
         )
+        if not selecting_match:
+            # On high-core-count VMs with managed IRQs (e.g. MANA NICs on v6
+            # SKUs), the kernel handles IRQ affinity and irqbalance correctly
+            # determines no rebalancing is needed. In this case, verify that
+            # irqbalance successfully completed at least one scan cycle by
+            # checking for interrupt enumeration output.
+            assert re.search(
+                r"Interrupt \d+ node_num",
+                result.stdout,
+            ), (
+                "irqbalance is not rebalancing irqs" + err_msg
+            )
+            log.debug(
+                "irqbalance did not select any IRQ for rebalancing "
+                "(likely managed IRQs on high-core-count VM), "
+                "but scan cycle completed successfully"
+            )
 
     @TestCaseMetadata(
         description="""