mantle/platform/qemu: Adjust dependencies on journal-stream unit

The unit kept getting killed when entering emergency.target, which
isn't very useful since some of the messages that hadn't been flushed
yet were the ones that revealed the root cause of the failure. Let's
set up the ordering/deps such that it doesn't get taken down when
entering emergency.target.

Also add a sleep 1 in the Destroy to allow for some remaining messages
to get flushed.

mantle/platform/qemu.go

@@ -288,6 +288,8 @@ func (inst *QemuInstance) Destroy() {
 		os.Remove(inst.qmpSocketPath) //nolint // Ignore Errors
 	}
 	if inst.journalPipe != nil {
+		plog.Debugf("Sleep 1 to allow for more journal messages to get flushed")
+		time.Sleep(1 * time.Second)
 		inst.journalPipe.Close()
 		inst.journalPipe = nil
 	}
@@ -1629,6 +1631,15 @@ func (builder *QemuBuilder) VirtioJournal(config *conf.Conf, queryArguments stri
 	var streamJournalUnit = fmt.Sprintf(`[Unit]
 	Requires=dev-virtio\\x2dports-mantlejournal.device
 	IgnoreOnIsolate=true
+	# DefaultDependencies=false so that Requires=sysinit.target
+	# won't be added to this unit, which would cause it to get
+	# taken down when isolating to emergency.target
+	DefaultDependencies=no
+	After=systemd.journal.service
+	# After systemd-journal-flush because otherwise the journalctl -f
+	# below will stop when the journal is flushed. Not sure if this is
+	# a bug or intended behavior.
+	After=systemd-journal-flush.service
 	[Service]
 	Type=simple
 	StandardOutput=file:/dev/virtio-ports/mantlejournal