fix name 'update_volume_status' to 'update_volume_stats'

_update_volume_status() private method is called by
get_volume_stats(). This is confusing as the volume's
stats and not status gets updated.

fixes bug #1199327

Change-Id: I9f7971e457250798320c14d3f58bdc199c8cfceb
Merge "fix name 'update_volume_status' to 'update_volume_stats'"