- If XEN_SLICE2IMAGE attribute is specified, create a 16GB partition 2
- Reduce the fraction of space used by the thin pool, especially on
  non-shared hosts.
- If xen-vg volume group already exists, use it. But also check to
  make sure thin-pool exists and create it if not.

The last change allows the user to specify a blockstore on the physical
host to use rather than having libvnode_xen build up its own.