Fix up calculating space info for mirrored volumes

Fix up calculating space info for mirrored volumes

The status reporting in the lvm driver for cinder volume incorrectly
reports free capacity. This is due to a couple of issues:
  a) the number of mirrors isn't taken into account
  b) there are some pathological cases where a
     simple division by total mirrors isn't sufficient
     because all mirrors must be on separate physical volumes.

Co-authored-by: Vishvananda Ishaya <vishvananda@gmail.com>
Co-authored-by: John Griffith <john.griffith@solidfire.com>
Closes-Bug: 1269964
Change-Id: I65e16b24367b4093a52c1c52d895fb58ef6a29ff