virsh: Split cmds to manage network filter from virsh.c

virsh: Split cmds to manage network filter from virsh.c

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