nwfilter: enable hex number inputs in filter XML

nwfilter: enable hex number inputs in filter XML

With this patch I want to enable hex number inputs in the filter XML. A
number that was entered as hex is also printed as hex unless a string
representing the meaning can be found.

I am also extending the schema and adding a test case. A problem with
the DSCP value is fixed on the way as well.

Changes from V1 to V2:

- using asHex boolean in all printf type of functions to select the
output format in hex or decimal format