allow name for VSH_OT_ARGV options

allow name for VSH_OT_ARGV options

A name will improve the usege, example

# virsh help echo
  NAME
    echo - echo arguments

  SYNOPSIS
    echo [--shell] [--xml] [<string>]...

  DESCRIPTION
    Echo back arguments, possibly with quoting.

  OPTIONS
    --shell          escape for shell use
    --xml            escape for XML use
    <string>         arguments to echo

"[<string>]..." is added to SYNOPSIS.
"<string>         arguments to echo" is added to OPTIONS.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>