Commands to manage network filter are moved from virsh.c to virsh-nwfilter.c,
with a few helpers for network filter command use.
* virsh.c: Remove network filter commands and a few helpers.
(vshCommandOptNWFilter, and vshCommandOptNWFilterBy)
* virsh-nwfilter.c: New file, filled with network filter commands and its helpers.
* po/POTFILES.in: Add virsh-nwfilter.c
* cfg.mk: Skip to check config.h including for virsh-nwfilter.c
