block: Remove 'backing': null from bs->{explicit_,}options

bs->options and bs->explicit_options shouldn't contain any options for
child nodes. bdrv_open_inherited() takes care to remove any options that
match a child name after opening the image and the same is done when
reopening.

However, we miss the case of 'backing': null, which is a child option,
but results in no child being created. This means that a 'backing': null
remains in bs->options and bs->explicit_options.

A typical use for 'backing': null is in live snapshots: blockdev-add for
the qcow2 overlay makes sure not to open the backing file (because it is
already opened and blockdev-snapshot will attach it). After doing a
blockdev-snapshot, bs->options and bs->explicit_options become
inconsistent with the actual state (bs has a backing file now, but the
options still say null). On the next occasion that the image is
reopened, e.g. switching it from read-write to read-only when another
snapshot is taken, the option will take effect again and the node
incorrectly loses its backing file.

Fix bdrv_open_inherited() to remove the 'backing' option from
bs->options and bs->explicit_options even for the case where it
specifies that no backing file is wanted.

Reported-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>

block.c

@@ -3019,6 +3019,8 @@ static BlockDriverState *bdrv_open_inherit(const char *filename,
                         "use \"backing\": null instead");
         }
         flags |= BDRV_O_NO_BACKING;
+        qdict_del(bs->explicit_options, "backing");
+        qdict_del(bs->options, "backing");
         qdict_del(options, "backing");
     }