Extend the filter XML to support priorities of chains

Extend the filter XML to support priorities of chains

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: Stefan Berger <stefanb@linux.vnet.ibm.com>