The virStorageTranslateDiskSourcePool method modifies a virDomainDiskDef
to resolve any storage pool reference. For some reason this was added
into the storage driver code, despite working entirely in terms of the
public APIs. Move it into the domain conf file and rename it to match the
object it modifies.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>