Don't swallow snapshot exceptions.

Fixes bug 949471

This raise is needed so that calling code can be notified to teardown
the failed snapshot.

Change-Id: I25a78e36b4bdb4ca52f1f999168a750dde134056

nova/virt/xenapi/vmops.py

@@ -696,7 +696,7 @@ def _create_snapshot(self, instance):
         except self.XenAPI.Failure, exc:
             LOG.error(_("Unable to Snapshot instance %(instance_uuid)s: "
                     "%(exc)s") % locals())
-            return
+            raise
 
     def _migrate_vhd(self, instance, vdi_uuid, dest, sr_path):
         instance_id = instance['id']