Rather than checking during XML processing, move the checks for correct
and valid auth into virDomainDiskDefParseValidate. This will introduce
virDomainDiskSourceDefParseAuthValidate to validate that the authdef
stored for the virStorageSource is valid. This can then be expanded
to service backingStore sources as well.

Alter the message text slightly as well to distinguish between an
unknown name and an incorrectly used name.  Since type is not a
mandatory field, add the NULLSTR() around the output of the unknown
error. NB, a config using unknown formatting would fail virschematest
since it only accepts 'iscsi' and 'ceph' as "valid" types.