qemu: snapshot: Don't modify persistent XML if disk source is different

While the VM is running the persistent source of a disk might differ
e.g. as the 'newDef' was redefined. Our snapshot code would blindly
rewrite the source of such disk if it shared the 'target'. Fix this by
checking whether the source is the same in the first place.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -15318,6 +15318,7 @@ qemuDomainSnapshotDiskDataCollect(virQEMUDriverPtr driver,
     qemuDomainSnapshotDiskDataPtr dd;
     char *backingStoreStr;
     virDomainSnapshotDefPtr snapdef = virDomainSnapshotObjGetDef(snap);
+    virDomainDiskDefPtr persistdisk;
     int ret = -1;
 
     if (VIR_ALLOC_N(data, snapdef->ndisks) < 0)
@@ -15338,13 +15339,12 @@ qemuDomainSnapshotDiskDataCollect(virQEMUDriverPtr driver,
         if (virStorageSourceInitChainElement(dd->src, dd->disk->src, false) < 0)
             goto cleanup;
 
-        /* Note that it's unsafe to assume that the disks in the persistent
-         * definition match up with the disks in the live definition just by
-         * checking that the target name is the same. We've done that
-         * historically this way though. */
+        /* modify disk in persistent definition only when the source is the same */
         if (vm->newDef &&
-            (dd->persistdisk = virDomainDiskByName(vm->newDef, dd->disk->dst,
-                                                   false))) {
+            (persistdisk = virDomainDiskByName(vm->newDef, dd->disk->dst, false)) &&
+            virStorageSourceIsSameLocation(dd->disk->src, persistdisk->src)) {
+
+            dd->persistdisk = persistdisk;
 
             if (!(dd->persistsrc = virStorageSourceCopy(dd->src, false)))
                 goto cleanup;