Fix indendation
Add lock around storage pool creation; closes #278
Change libvirt method for creating storage pool

We should use define_storage pool_xml instead of create_storage_pool_xml
so that

* the pool is permanent
* pool creation succeeds if the directory does not already exist
Add lock around base box upload

I'm not entirely happy with this, because we only really need a lock
when multiple VMs are using the same base box. If they are using
different boxes, we could safely upload in parallel. I did not see an
obvious way to implement that, so this will work. Parallel uploads are
likely not a performance improvement, anyway.
Merge pull request #281 from sciurus/fix/lock-pool-creation

Fixes for storage pool creation and base box uploads