Privacy: correct field name on list save

A wrong field name was used here (propably due to refactor)
Caused "block elements" not to be saved.
```xml
<list xmlns="jabber:iq:privacy" name="invisible"><item action="deny" order="1"/></list>
```
instead of:
```xml
<list xmlns="jabber:iq:privacy" name="invisible"><item action="deny" order="1"><presence-out/></item></list>
```