Tidy error message.

Signed-off-by: Ewan Mellor <ewan@xensource.com>

tools/python/xen/xend/XendDomainInfo.py

@@ -1709,8 +1709,10 @@ class XendDomainInfo:
 
         dom =  XendDomain.instance().domain_lookup_nr(name)
         if dom and dom.info['uuid'] != self.info['uuid']:
-            raise VmError("VM name '%s' already exists as domain %s" %
-                          (name, str(dom.domid)))
+            raise VmError("VM name '%s' already exists%s" %
+                          (name,
+                           dom.domid is not None and
+                           ("as domain %s" % str(dom.domid)) or ""))
         
 
     def update(self, info = None, refresh = True):