filter 0-device-weight when:

  - getting blkio parameters with --config
  - starting up a domain

When testing with blkio, I found these issues:

  (dom is down)
  virsh blkiotune dom --device-weights /dev/sda,300,/dev/sdb,500
  virsh blkiotune dom --device-weights /dev/sda,300,/dev/sdb,0
  virsh blkiotune dom
  weight         : 800
  device_weight  : /dev/sda,200,/dev/sdb,0

  # issue 1: shows 0 device weight of /dev/sdb that may confuse user

  (continued)
  virsh start dom

  # issue 2: If /dev/sdb doesn't exist, libvirt refuses to bring the
  # dom up because it wants to set the device weight to 0 of a
  # non-existing device. Since 0 means no weight-limit, we really don't
  # have to set it.