Catch an exception in power off procedure

In libvirt driver, an exception of libvirt error may be thrown
when calling shutdown method.

This exception is caught by nobody, then the following retry
logic and the destroy method will be missed, causing the power
off procedure failure.

Catch this exception (as the retry logic does) to make the
procedure go on.

Change-Id: Icf9b15207a7e13333cf5640a5bce0e1082ecf4ec

Author: CDream

nova/virt/libvirt/driver.py

@@ -3876,7 +3876,12 @@ def _clean_shutdown(self, instance, timeout, retry_interval):
 
         LOG.debug("Shutting down instance from state %s", state,
                   instance=instance)
-        guest.shutdown()
+        try:
+            guest.shutdown()
+        except libvirt.libvirtError as e:
+            LOG.debug("Ignoring libvirt exception from shutdown request: %s",
+                      encodeutils.exception_to_unicode(e),
+                      instance=instance)
         retry_countdown = retry_interval
 
         for sec in range(timeout):