virsh: Split cmds to manage network from virsh.c

virsh: Split cmds to manage network from virsh.c

Commands to manage network are moved from virsh.c to virsh-network.c,
with a few helpers for network command use.

* virsh.c: Remove network commands and a few helpers.
* virsh-network.c: New file, filled with network commands and its
                   helpers.
* po/POTFILES.in: Add virsh-network.c
* cfg.mk: Skip to check config.h including for virsh-network.c