In stageuser plugin, option to move a preserved user to staging container does not work.

Steps to reproduce:
1. create a preserved user:
# ipa user-add tuser --first test --last user

# ipa user-del tuser --preserve

1. try to restore the user (move it to staging container):
   # ipa stageuser-add tuser --from-delete

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Expected result:
User is now a staged user and is no longer in preserved container.

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Actual result:
Asks for specifying first and last name and then fails: ERROR: Internal error occured.

The user is still in preserved container, no staged user is created.