Allow power_off when instance doesn't exist

Certain errors that can occur while booting an instance may leave no
VM on the Xen hypervisor.  This can make that instance undeletable,
since the first step in soft_delete() is to power_off the instance,
and nothing catches the NotFound error thrown if the instance doesn't
exist.  The libvirt support handles the case of a non-existant
instance by ignoring NotFound--after all, a non-existant instance is,
by definition, powered off, right?

Fixes bug 1029132.

Change-Id: I5bffe3b61fe92fc32fb70cab941fa5ecd250df77

nova/virt/xenapi/vmops.py

@@ -1089,8 +1089,12 @@ def unrescue(self, instance):
 
     def power_off(self, instance):
         """Power off the specified instance."""
-        vm_ref = self._get_vm_opaque_ref(instance)
-        vm_utils.shutdown_vm(self._session, instance, vm_ref, hard=True)
+        try:
+            vm_ref = self._get_vm_opaque_ref(instance)
+            vm_utils.shutdown_vm(self._session, instance, vm_ref, hard=True)
+        except exception.NotFound:
+            LOG.warning(_("VM is not present, skipping power off..."),
+                        instance=instance)
 
     def power_on(self, instance):
         """Power on the specified instance."""