Create symlinks to intances' block devices

Create symlinks to intances' block devices

This is a forward-port of commit 1148 on the 1.2 branch:
  Change the _GatherBlockDevs private function, called only one time by
  StartInstance, to _GatherAndLinkBlockDevs, and make it transform the
  device returned even more by calling the new _SimlinkBlockDev auxiliary
  function.

  This makes sure that every time an instance is started symlinks to its
  block devices are created, and the instance is started off them, rather
  than the underlying block devices.

  Reviewed-by: iustinp

The changes we make to the patch is related to newer function signatures
in 2.0, and to the fact that iv_name is deprecated and we use instead
disk%d based on the disk index.

Original-Author: ultrotter