tests: do not use lazy unmount

If unmount is going to fail, let it do so explicitly, instead of hiding
the failure now, and observing it later at rmdir.
And if it fails, lets report what process is using that mount point.

qubes/tests/storage_reflink.py

@@ -131,7 +131,11 @@ def mkdir_fs(directory, fs_type,
 
 def rmtree_fs(directory):
     if os.path.ismount(directory):
-        cmd('sudo', 'umount', '-l', directory)
+        try:
+            cmd('sudo', 'umount', directory)
+        except:
+            cmd('sudo', 'fuser', '-vm', directory)
+            raise
         # loop device and backing file are garbage collected automatically
     cmd('sudo', 'chattr', '-i', directory)
     cmd('sudo', 'chmod', '777', directory)