handle type=user-id register and unregister response messages.
suggested and initial diff by btorresgil.

$ cat t/unreg.xml
<uid-message>
  <version>2.0</version>
  <type>update</type>
  <payload>
    <unregister>
      <entry identifier="tag1" ip="10.0.0.3"/>
      <entry identifier="tag2" ip="10.0.0.3"/>
      <entry identifier="tag1" ip="10.0.0.4"/>
      <entry identifier="tag3" ip="xxx"/>
    </unregister>
  </payload>
</uid-message>

$ ./panxapi.py -t pa-200 -U t/unreg.xml
dynamic-update: error: "message: Invalid IP address ip: xxx"