This patch extends the filter XML to support priorities of chains
in the XML. An example would be:

<filter name='allow-arpxyz' chain='arp-xyz' priority='200'>
[...]
</filter>

The permitted values for priorities are [-1000, 1000].
By setting the priority of a chain the order in which it is accessed
from the interface root chain can be influenced.
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>