Express a properly terminated backing chain by putting a
virStorageSource of type VIR_STORAGE_TYPE_NONE in the chain. The newly
used helpers simplify this greatly.
The change fixes a bug as formatting an incomplete backing chain and
parsing it back would end up in expressing a terminated chain since
src->backingStoreRaw was not populated. By relying on the terminator
object this can be now processed appropriately.
This way we can safely differentiate what XMLs contain whole domain
definitions and which contain just devices. Thanks to that we can
test the domain XMLs in virschematest again.
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>