migration: Fix coding style (whitespace issues)

* Remove trailing whitespace (fixes 9 errors from checkpatch.pl).
  One comment line was longer than 80 characters, so wrap it
  and fix a typo, too.
* Replace tabs by blanks (fixes 1 error).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
