storage: Fix incorrect format for <disk> <auth> XML

https://bugzilla.redhat.com/show_bug.cgi?id=1256999

After creating a copy of the 'authdef' in a pool -> disk translation,
unconditionally clear the 'authType' in the resulting disk auth def
structure since that's used for a storage pool and not a disk.  This
ensures virStorageAuthDefFormat will properly format the <auth> XML
for a <disk> (e.g. it won't have a <auth type='%s'.../>).

src/storage/storage_driver.c

@@ -3199,6 +3199,8 @@ virStorageTranslateDiskSourcePoolAuth(virDomainDiskDefPtr def,
     def->src->auth = virStorageAuthDefCopy(source->auth);
     if (!def->src->auth)
         goto cleanup;
+    /* A <disk> doesn't use <auth type='%s', so clear that out for the disk */
+    def->src->auth->authType = VIR_STORAGE_AUTH_TYPE_NONE;
     ret = 0;
 
  cleanup: