qvm-template-postprocess: call fstrim after removing image file

This is especially important on LVM thin pool, where space after
removing the file needs to be given back to the pool, to be reused for
other volumes (for example this template).

qubesadmin/tools/qvm_template_postprocess.py

@@ -214,6 +214,11 @@ def post_install(args):
 
     if not args.keep_source:
         shutil.rmtree(args.dir)
+        # if running as root, tell underlying storage layer about just freed
+        # data blocks
+        if os.getuid() == 0:
+            subprocess.call(['sync', '-f', os.path.dirname(args.dir)])
+            subprocess.call(['fstrim', os.path.dirname(args.dir)])
 
     return 0