Round the recommended thpool metadata size to extents
Round the recommended thpool metadata size to extents

Otherwise LVM does it when creating the thin pool and we may end
up with not enough space in the VG to create the data part of the
pool (which happens afterwards).

Resolves: rhbz#1456528

(port of the commit 062360ce6498ee33b62a7ee6833e681d274522b4 from
the rhel7-branch)

rvykydal/blivet,rvykydal/blivet,vojtechtrefny/blivet,rhinstaller/blivet,rhinstaller/blivet,vojtechtrefny/blivet,jkonecny12/blivet,jkonecny12/blivet