storage: Create common file/dir pool backend helpers

storage: Create common file/dir pool backend helpers

Move some pool functions to storage_util to create local/common helpers
using the same naming syntax as the existing upload, download, and wipe
virStorageBackend*Local API's.

In the process of doing so, found a few API's that can now become local
to storage_util. In order to distinguish between local/external - I
changed the names of the now local only ones from "virStorageBackend..."
to just "storageBackend..."

Signed-off-by: John Ferlan <jferlan@redhat.com>