devshell: fix journal monitoring

Commit 7b07ffe dropped the `-o json` argument to the journalctl query
that the devshell code uses to monitor system boot and know when to SSH.
Fix this by re-adding the argument but only for devshell so that the
default is still to output traditional logs (as in the testiso case).

Fixes 7b07ffe ("mantle/platform/qemu: drop json formatting from
journal output").

mantle/cmd/kola/devshell.go

@@ -430,7 +430,7 @@ func watchJournal(builder *platform.QemuBuilder, conf *conf.Conf, stateChan chan
 		},
 	}
 
-	r, err := builder.VirtioJournal(conf, "--system")
+	r, err := builder.VirtioJournal(conf, "-o json --system")
 	if err != nil {
 		return err
 	}