This patch improves the processing of storage information
as result of the 'node_info' call in the following way:
- It removes the override for LVM storage. This was needed
  in MakeLegacyNode info to be compatible to the
  iallocator, which no longer uses it.
- When no storage information is available (for example
  when only disk templates are enabled, for which no
  storage reporting exists), then this is handled more
  gracefully.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>