Added class to contain information about a PV

This makes the code more readable and easier to upgrade.
bdev.LogicalVolume.GetPVInfo and the code that depends on it have been
refactored to use the new class.

utils.CheckVolumeGroupSize() has been moved to lib/utils/lvm.py, where more
functions will be added.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>