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

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'.../>).

(cherry picked from commit 5275c0f4a1267c630b5c3b77fac8cbe4fcaa3adb)