Terminate backing chains explicitly

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.

tests/qemuhotplugtestdomains/qemuhotplug-base-ccw-live-with-ccw-virtio.xml

@@ -22,7 +22,6 @@
     <disk type='file' device='disk'>
       <driver name='qemu' type='raw' cache='none'/>
       <source file='/dev/null'/>
-      <backingStore/>
       <target dev='vde' bus='virtio'/>
       <readonly/>
       <shareable/>