Remove instance's symlinks

This is a forward-port of commits 1150 and 1151 on the 1.2 branch:
  Add _RemoveBlockDevLinks auxiliary function, called when an instance
  fails to start and when it is shut down.

  Reviewed-by: iustinp

and:
  Fix cut&paste error when removing symlinks

  It's just whitespace... isn't it? uhm... :) Anyway, fixing an error made
  when reformatting the code for the new "safer" behaviour.

  Reviewed-by: iustinp

Original-Author: ultrotter