Since the virStorageEncryptionPtr encryption; is a member of
 _virStorageSource it really should be allowed to be a subelement
of the disk <source> for various disk formats:

   Source{File|Dir|Block|Volume}
   SourceProtocol{RBD|ISCSI|NBD|Gluster|Simple|HTTP}

NB: Simple includes sheepdog, ftp, ftps, tftp

That way we can set up to allow the <encryption> element to be
formatted within the disk source, but we still need to be wary
from whence the element was read - see keep track and when it
comes to format the data, ensure it's written in the correct place.

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

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