Fix gnt-debug iallocator

There was an issue with the recent ipolicy introduction which lead to a
bug in gnt-debug iallocator. It was not providing the spindle_use field
and therefore it wont let you create a valid iallocator request.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>