libvirt: add debug logging in detach_device_with_retry

When we hit failures in this method it can be hard to
sort out from a traceback what actually happened, and
if we failed from the intial detach or if we failed
during the retry loop.

This patch adds some simple debug logging statements
to help give context to what's going on when we need
to debug a failure in this operation.

Change-Id: I392192f139ced077664e54469eead0b0df9ebf4a
Related-Bug: #1673483
(cherry picked from commit 33d0b029f1)

nova/virt/libvirt/guest.py

@@ -395,6 +395,9 @@ class Guest(object):
             # Raise DeviceNotFound if the device isn't found during detach
             try:
                 self.detach_device(conf, persistent=persistent, live=live)
+                LOG.debug('Successfully detached device %s from guest. '
+                          'Persistent? %s. Live? %s',
+                          device, persistent, live)
             except libvirt.libvirtError as ex:
                 with excutils.save_and_reraise_exception():
                     errcode = ex.get_error_code()
@@ -417,7 +420,9 @@ class Guest(object):
         if conf is None:
             raise exception.DeviceNotFound(device=alternative_device_name)
 
+        LOG.debug('Attempting initial detach for device %s', device)
         _try_detach_device(conf, persistent, live)
+        LOG.debug('Start retrying detach until device %s is gone.', device)
 
         @loopingcall.RetryDecorator(max_retry_count=max_retry_count,
                                     inc_sleep_time=inc_sleep_time,