block/drive-mirror: Reuse backing HD for sync=none

For "none" sync mode in "absolute-paths" mode, the current image should
be used as the backing file for the newly created image.

The current behavior is:
a) If the image to be mirrored has a backing file, use that (which is
   wrong, since the operations recorded by "none" are applied to the
   image itself, not to its backing file).
b) If the image to be mirrored lacks a backing file, the target doesn't
   have one either (which is not really wrong, but not really right,
   either; "none" records a set of operations executed on the image
   file, therefore having no backing file to apply these operations on
   seems rather pointless).

For a, this is clearly a bugfix. For b, it is still a bugfix, although
it might break existing API - but since that case crashed qemu just
three weeks ago (before 1452686), we
can safely assume there is no such API relying on that case yet.

Suggested-by: Paolo Bonzini <[email protected]>
Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-id: [email protected]
Signed-off-by: Anthony Liguori <[email protected]>

blockdev.c

@@ -2021,6 +2021,9 @@ void qmp_drive_mirror(const char *device, const char *target,
     if (!source && sync == MIRROR_SYNC_MODE_TOP) {
         sync = MIRROR_SYNC_MODE_FULL;
     }
+    if (sync == MIRROR_SYNC_MODE_NONE) {
+        source = bs;
+    }
 
     size = bdrv_getlength(bs);
     if (size < 0) {