Fix crash when calling screendump from monitor.

This was a regression introduced with r6839:
DisplayAllocator interface (Stefano Stabellini)

Signed-off-by: Stefan Weil <weil@mail.berlios.de>

Index: trunk/hw/vga.c
===================================================================
--- trunk.orig/hw/vga.c	2009-03-24 19:35:00.000000000 +0100
+++ trunk/hw/vga.c	2009-03-24 19:37:56.000000000 +0100
@@ -2637,6 +2637,7 @@
     dcl.dpy_resize = vga_save_dpy_resize;
     dcl.dpy_refresh = vga_save_dpy_refresh;
     register_displaychangelistener(ds, &dcl);
+    ds->allocator = &default_allocator;
     ds->surface = qemu_create_displaysurface(ds, w, h);
 
     s->ds = ds;