Simplify hypervisor block_devices structure

This is a partial forward-port of commit 1136 on the 1.2 branch:

  The hypervisor doesn't need to be passed the whole block device
  structure, so we'll just give it the block device name on the local
  node, and the name as seen by the instance. This will make it easier to
  manipulate it later without messing with the block devices (eg. by
  changing the system name to a symlink to the name itself).

  Since the HVM hypervisor changes the "virtual" name a note is added
  calling for a redesign that doesn't need this change, as different
  hypervisors and emulation types will anyway have different names for
  exported devices.

  Reviewed-by: iustinp

The changes in this patch compared to the original are:
  - we keep passing the original disk object, not for its iv_name, but
    for it's physical_id which is needed by the file driver (this could
    be fixed maybe)
  - we don't use the iv_name anymore, since in 2.0 we already use the
    index of the device

Original-Author: ultrotter