[dbus-xml] serialize bitmap data as comma separated lists

```
For improved readability, serialize bitmap constrained data as delimiter
separated lists. We accept the following forms in intf.xml files:

  <node>flag1__DELIM__...__DELIM__flagN</node>

where __DELIM__ is " ,|\t\n"

OR

  <node>
    <flag1/>
    ...
    <flagN/>
  </node>

When dispalying this data to user (via show-xml, for example), we use
the form:

  <node>flag1 ,..., flagN</node>
```