Fix propagation of storage parameters to/from backend

Fix propagation of storage parameters to/from backend

This patch fixes two problems with the storage reporting
that showed up in the QA for exclusive storage:

- The processing of storage space information for instance
  operations wrongly assumed that the volume group's
  storage information is always the first in the list.
- The storage parameter 'exclusive storage' was not
  correctly extracted from the list of storage parameters.
- There was a bug in the preparation of storage unit for
  the node info call in the iallocator. The exclusive
  storage flag was not set for spindles and the format
  of the storage parameters for LVM vgs was a boolean
  and not a list.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>