util: storage: Copy driver type when initializing chain element

virStorageSourceInitChainElement initializes a new storage chain element
for use as a new disk source. If the new element doesn't contain the
driver name, copy it from the old source.

This fixes issue where a disk would forget the driver after a snapshot.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1140984

src/util/virstoragefile.c

@@ -1927,6 +1927,10 @@ virStorageSourceInitChainElement(virStorageSourcePtr newelem,
         virStorageSourceSeclabelsCopy(newelem, old) < 0)
         goto cleanup;
 
+    if (!newelem->driverName &&
+        VIR_STRDUP(newelem->driverName, old->driverName) < 0)
+        goto cleanup;
+
     newelem->shared = old->shared;
     newelem->readonly = old->readonly;