Extend NWFilter parameter parser to cope with lists of values

Extend NWFilter parameter parser to cope with lists of values

This patch modifies the NWFilter parameter parser to support multiple
elements with the same name and to internally build a list of items.
An example of the XML looks like this:

        <parameter name='TEST' value='10.1.2.3'/>
        <parameter name='TEST' value='10.2.3.4'/>
        <parameter name='TEST' value='10.1.1.1'/>

The list of values is then stored in the newly introduced data type
virNWFilterVarValue.

The XML formatter is also adapted to print out all items in alphabetical
order sorted by 'name'.

This patch also fixes a bug in the XML schema on the way.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>