storage_driver: Resolve Coverity REVERSE_INULL

storage_driver: Resolve Coverity REVERSE_INULL

There were two occurrances of attempting to initialize actualType by
calling virStorageSourceGetActualType(src) prior to a check if (!src)
resulting in Coverity complaining about the possible NULL dereference
in virStorageSourceGetActualType() of src.

Resolve by moving the actualType setting until after checking !src