The QEMU smoketest harness sometimes aborts with a timeout even though the image under test is fine, mostly on loaded CI hosts or with large images.

After installation the harness navigates the GRUB menus to select the serial console. Under host load a key press can arrive split or get dropped, so GRUB leaves the submenu and boots the default entry. The harness kept waiting for the serial console entry and aborted the whole run. Such a timeout is now logged as a warning and the default entry, which already uses the serial console chosen during install, continues to boot. A VM that is really stuck in GRUB is still caught by the following login check.

Installing GRUB and flushing the image to disk can also take longer than the default 60 second timeout, so a working install was reported as failed. This wait is now raised to 300 seconds.