stubdom: make sure to use old tree for now

diff -r 70a43a82aaf1 stubdom/Makefile
@@ -3,12 +3,12 @@
 
 export XEN_OS=MiniOS
 
-CONFIG_QEMU=ioemu
-
 export stubdom=y
 export debug=y
 NEWLIB_CFLAGS=-DDEBUG=1
 include $(XEN_ROOT)/Config.mk
+
+override CONFIG_QEMU=ioemu
 
 IOEMU_OPTIONS=--disable-sdl --disable-opengl --disable-vnc-tls --disable-brlapi --disable-kqemu
 ZLIB_VERSION=1.2.3