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
Merge "Fix up calculating space info for mirrored volumes"