nwfilter: fix tear down order and consolidate functions

nwfilter: fix tear down order and consolidate functions

To avoid race-conditions, the tear down of a filter has to happen before
the tap interface disappears and another tap interface with the same
name can re-appear. This patch tries to fix this. In one place, where
communication with the qemu monitor may fail, I am only tearing the
filters down after knowing that the function did not fail.

I am also moving the tear down functions into an include file for other
drivers to reuse.