PAE must be on for 64-on-64 to work at all.

Index: xen-unstable/tools/python/xen/xend/image.py
===================================================================
--- xen-unstable.orig/tools/python/xen/xend/image.py
+++ xen-unstable/tools/python/xen/xend/image.py
@@ -265,7 +265,7 @@ class HVMImageHandler(ImageHandler):
 
         self.pid = None
 
-        self.pae  = int(vmConfig['platform'].get('pae',  0))
+        self.pae  = int(vmConfig['platform'].get('pae',  1))
         self.apic = int(vmConfig['platform'].get('apic', 0))
         self.acpi = int(vmConfig['platform'].get('acpi', 0))