Since the virStorageAuthDefPtr auth; is a member of _virStorageSource
it really should be allowed to be a subelement of the disk <source>
for the RBD and iSCSI prototcols. That way we can set up to allow
the <auth> element to be formatted within the disk source.

Since we've allowed the <auth> to be a child of <disk>, we'll need
to keep track of how it was read so that when writing out we'll know
whether to format as child of <disk> or <source>. For the argv2xml
parsing, let's format under <source> as a preference. Do not allow
<auth> to be both a child of <disk> and <source>.

Modify the qemuxml2argvtest to add a parse failure when there is an
<auth> as a child of <disk> *and* an <auth> as a child of <source>.

Add tests to validate that if the <auth> was found in <source>, then
the resulting xml2xml and xml2arg works just fine.  The two new .args
file are exact copies of the non "-source" version of the file.

The virschematest will read the new test files and validate from a
RNG viewpoint things are fine

Update the virstoragefile, virstoragetest, and args2xml file to show
the "preference" to place <auth> as a child of <source>.