Fix available space calc for mirrored volumes

When mirroring is in use, we need a more complicated process for
determining the actual available space. There are some situations where
this is more complex than a simple total / mirrors because extents
for each mirror have to be on separate drives.

Change-Id: I75af4053e8a8bd49134bdbbed0e7276395e366b6
Closes-bug: #1269964