xenapi: cached images should be cleaned up by time

For xenapi driver, there needs to be some way to delete cached
images based on when they were created. add an optional arg to
control delete operation.

Change-Id: I24fc45e989aa951aab55a261fce77f7e3667d988
Closes-bug: 1481689

tools/xenserver/destroy_cached_images.py

@@ -21,6 +21,8 @@ Options:
     --dry_run - Don't actually destroy the VDIs
     --all_cached - Destroy all cached images instead of just unused cached
                    images.
+    --keep_days - N - Only remove those cached images which were created
+                      more than N days ago.
 """
 import eventlet
 eventlet.monkey_patch()
@@ -52,7 +54,11 @@ destroy_opts = [
                      ' images.'),
     cfg.BoolOpt('dry_run',
                 default=False,
-                help='Don\'t actually delete the VDIs.')
+                help='Don\'t actually delete the VDIs.'),
+    cfg.IntOpt('keep_days',
+               default=0,
+               help='Destroy cached images which were'
+                    ' created over keep_days.')
 ]
 
 CONF = nova.conf.CONF
@@ -69,7 +75,7 @@ def main():
     sr_ref = vm_utils.safe_find_sr(_session)
     destroyed = vm_utils.destroy_cached_images(
         _session, sr_ref, all_cached=CONF.all_cached,
-        dry_run=CONF.dry_run)
+        dry_run=CONF.dry_run, keep_days=CONF.keep_days)
 
     if '--verbose' in sys.argv:
         print('\n'.join(destroyed))