storage: Don't adjust pool alloc/avail values for disk backend
    
    Commit id '2ac0e647' for https://bugzilla.redhat.com/show_bug.cgi?id=1206521
    was meant to be a generic check for the CreateVol, CreateVolFrom, and
    DeleteVol paths to check if the storage backend's changed the pool's view
    of allocation or available values.
    
    Unfortunately as it turns out this caused a side effect when the disk backend
    created an extended partition there would be no actual storage removed from
    the pool, thus the changes would not find any change in allocation or
    available and incorrectly update the pool values using the size of the
    extended partition. A subsequent refresh of the pool would reset the
    values appropriately.
    
    This patch modifies those checks in order to specifically not update the
    pool allocation and available for only the disk backend rather than be
    generic before and after checks.