storage: Determine the local storage type right away

storage: Determine the local storage type right away

When walking the backing chain we previously set the storage type to
_FILE and let the virStorageFileGetMetadataFromFDInternal update it to
the correct type later on.

This patch moves the actual storage type determination to the place
where we parse the backing store name so that the code can later be
switched to use virStorageFileReadHeader() directly.