Capture redacted guest serial while waiting for shutdown

dev/iso-smoke.py

@@ -575,10 +575,35 @@ class VM:
             raise
 
     def wait_poweroff(self, timeout):
-        try:
-            rc = self.process.wait(timeout=timeout)
-        except subprocess.TimeoutExpired as exc:
-            raise SmokeError('Guest failed to power off in time') from exc
+        deadline = time.monotonic() + timeout
+        serial_open = True
+        while self.process.poll() is None:
+            remaining = deadline - time.monotonic()
+            if remaining <= 0:
+                raise SmokeError('Guest failed to power off in time')
+            if serial_open:
+                try:
+                    self.console.read(min(remaining, 0.25))
+                except SmokeError as exc:
+                    if str(exc) != 'Guest console closed':
+                        raise
+                    serial_open = False
+            else:
+                try:
+                    self.process.wait(timeout=remaining)
+                except subprocess.TimeoutExpired as exc:
+                    raise SmokeError('Guest failed to power off in time') from exc
+        # Capture bytes queued immediately before exit, even if poll observed
+        # the child exiting before the first serial read.
+        while serial_open:
+            try:
+                if not self.console.read(0):
+                    break
+            except SmokeError as exc:
+                if str(exc) != 'Guest console closed':
+                    raise
+                serial_open = False
+        rc = self.process.returncode
         if rc:
             raise SmokeError(f'Test QEMU failed during poweroff ({rc})')