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/sexpr2xmldata/sexpr2xml-pci-devs.xml

@@ -18,7 +18,6 @@
     <disk type='block' device='disk'>
       <driver name='phy'/>
       <source dev='/dev/MainVG/GuestVG'/>
-      <backingStore/>
       <target dev='xvda' bus='xen'/>
     </disk>
     <console type='pty'>