xend: fix dual destroy

After changeset 18030 and 18064 were checked in, I found some issues
when creating HVM domains with devices assigned.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>

tools/python/xen/xend/XendDomainInfo.py

@@ -2374,6 +2374,9 @@ class XendDomainInfo:
     def destroy(self):
         """Cleanup VM and destroy domain.  Nothrow guarantee."""
 
+        if self.domid is None:
+            return
+
         log.debug("XendDomainInfo.destroy: domid=%s", str(self.domid))
 
         paths = self._prepare_phantom_paths()