virsh: Split cmds to manage host interface from virsh.c

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

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

po/POTFILES.in

@@ -180,6 +180,7 @@ tools/virsh.c
 tools/virsh-domain-monitor.c
 tools/virsh-domain.c
 tools/virsh-edit.c
+tools/virsh-interface.c
 tools/virsh-network.c
 tools/virsh-pool.c
 tools/virsh-volume.c