PaaS model machine icons are not saved

Reproduction:
- File > New, select PaaS model
- Add machine, with any attribute values
- Save model
- Resulting xml file does not contain an 'icon' element

For example:
```
<?xml version="1.0" encoding="ISO-8859-1" standalone="no"?>
<!DOCTYPE system SYSTEM "iSPD.dtd">
<system version="2.3">
    <owner id="user1" powerlimit="100.0"/>
</system>
```