This patch provides unit tests for _GetVgSpindlesInfo.
In order to mock the used function for the 'bdev' module
I made the low-level storage function an optional parameter
of _GetVgSpindlesInfo. I also found out that mocked
module functions stay mocked even after the execution
of the text finished. Fixed this for one occurrence. More
in the next patch.

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