block: Reuse bs as backing hd for drive-backup sync=none

Opening the backing image for the second time is bad, especially here
when it is also in use as the active image as the source. The
drive-backup job itself doesn't read from target->backing for COW,
instead it gets data from the write notifier, so it's not a big problem.
However, exporting the target to NBD etc. won't work, because of the
likely stale metadata cache.

Use BDRV_O_NO_BACKING in this case and manually set up the backing
BdrvChild.

Cc: qemu-stable@nongnu.org
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit fc0932fd)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

blockdev.c

@@ -3170,6 +3170,7 @@ static BlockJob *do_drive_backup(DriveBackup *backup, BlockJobTxn *txn,
     Error *local_err = NULL;
     int flags;
     int64_t size;
+    bool set_backing_hd = false;
 
     if (!backup->has_speed) {
         backup->speed = 0;
@@ -3220,6 +3221,8 @@ static BlockJob *do_drive_backup(DriveBackup *backup, BlockJobTxn *txn,
     }
     if (backup->sync == MIRROR_SYNC_MODE_NONE) {
         source = bs;
+        flags |= BDRV_O_NO_BACKING;
+        set_backing_hd = true;
     }
 
     size = bdrv_getlength(bs);
@@ -3246,7 +3249,9 @@ static BlockJob *do_drive_backup(DriveBackup *backup, BlockJobTxn *txn,
     }
 
     if (backup->format) {
-        options = qdict_new();
+        if (!options) {
+            options = qdict_new();
+        }
         qdict_put_str(options, "driver", backup->format);
     }
 
@@ -3257,6 +3262,14 @@ static BlockJob *do_drive_backup(DriveBackup *backup, BlockJobTxn *txn,
 
     bdrv_set_aio_context(target_bs, aio_context);
 
+    if (set_backing_hd) {
+        bdrv_set_backing_hd(target_bs, source, &local_err);
+        if (local_err) {
+            bdrv_unref(target_bs);
+            goto out;
+        }
+    }
+
     if (backup->has_bitmap) {
         bmap = bdrv_find_dirty_bitmap(bs, backup->bitmap);
         if (!bmap) {