virsh: Split cmds of storage volume group from virsh.c

virsh: Split cmds of storage volume group from virsh.c

This splits commands of storage volume group into virsh-volume.c,
The helpers not for common use are moved too. Standard copyright
is added for the new file.

* tools/virsh.c:
  Remove commands for storage storage volume and a few helpers.
  (vshCommandOptVol, vshCommandOptVolBy).

* tools/virsh-volume.c:
  New file, filled with commands of storage volume group and its
  helpers.

* po/POTFILES.in:
  Add virsh-volume.c

* cfg.mk:
  Skip to check config.h including for virsh-volume.c