stubdom: fix copy/paste typo in select()

diff -r c7a50d4a3adc extras/mini-os/lib/sys.c
@@ -734,7 +734,7 @@
 	    FD_CLR(i, exceptfds);
 	    break;
 	case FTYPE_CONSOLE:
-	    if (FD_ISSET(i, writefds)) {
+	    if (FD_ISSET(i, readfds)) {
                 if (xencons_ring_avail())
 		    n++;
 		else