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

For improved readability, serialize bitmap constrained data as comma
separated lists. We accept the following forms in intf.xml files:
  <node>flag1,...,flagN</node>
OR
  <node>
    <flag1/>
    ...
    <flagN/>
  </node>

When dispalying this data to user (via show-xml, for example), we use
the form:
  <node>flag1,...,flagN</node>