This issue is caused by a resource leak due to a missing FileInputStream.close().

Here is the pull request with the fix:
https://github.com/hibernate/hibernate-orm/pull/876

And here's the plugin log for the test project after the fix:

[INFO] Started enhance plugin.....
[INFO] Processing Composite class file [hbm.test.Address].
[INFO] Processing Entity class file [hbm.test.Director].
[INFO] Processing Entity class file [hbm.test.Movie].
[INFO] Processing Entity class file [hbm.test.Theatre].
[INFO] Enhance plugin completed.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------