lvconvert: fix --repair after vgreduce

lvconvert: fix --repair after vgreduce

In case N images fail (N <= parity chunks) _and_
a "vgreduce --removemissing --force VG" was applied
a following repair of the RaidLV fails:

  Unable to remove N images:  Only 0 devices given.
  Failed to remove the specified images from tb/r.
  Failed to replace faulty devices in tb/r.

Fix as of this commit results in correct repair:

  Faulty devices in tb/r successfully replaced.