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