Fix extent size issue when creating thin pool.

The LVM create thin pool rounds up extents, this
change rounds the free space down to an int to avoid
this issue and also cleans up some comments and
code around how PV's are set up and used.

Change-Id: I725aa819c21c38f464595d0146d6640ecd1e6440
Merge "Fix extent size issue when creating thin pool."