block: Remove child references from bs->{options,explicit_options}

Block drivers allow opening their children using a reference to an
existing BlockDriverState. These references remain stored in the
'options' and 'explicit_options' QDicts, but we don't need to keep
them once everything is open.

What is more important, these values can become wrong if the children
change:

    $ qemu-img create -f qcow2 hd0.qcow2 10M
    $ qemu-img create -f qcow2 hd1.qcow2 10M
    $ qemu-img create -f qcow2 hd2.qcow2 10M
    $ $QEMU -drive if=none,file=hd0.qcow2,node-name=hd0 \
            -drive if=none,file=hd1.qcow2,node-name=hd1,backing=hd0 \
            -drive file=hd2.qcow2,node-name=hd2,backing=hd1

After this hd2 has hd1 as its backing file. Now let's remove it using
block_stream:

    (qemu) block_stream hd2 0 hd0.qcow2

Now hd0 is the backing file of hd2, but hd2's options QDicts still
contain backing=hd1.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -2763,12 +2763,15 @@ static BlockDriverState *bdrv_open_inherit(const char *filename,
         }
     }
 
-    /* Remove all children options from bs->options and bs->explicit_options */
+    /* Remove all children options and references
+     * from bs->options and bs->explicit_options */
     QLIST_FOREACH(child, &bs->children, next) {
         char *child_key_dot;
         child_key_dot = g_strdup_printf("%s.", child->name);
         qdict_extract_subqdict(bs->explicit_options, NULL, child_key_dot);
         qdict_extract_subqdict(bs->options, NULL, child_key_dot);
+        qdict_del(bs->explicit_options, child->name);
+        qdict_del(bs->options, child->name);
         g_free(child_key_dot);
     }
 
@@ -3290,6 +3293,7 @@ void bdrv_reopen_commit(BDRVReopenState *reopen_state)
 {
     BlockDriver *drv;
     BlockDriverState *bs;
+    BdrvChild *child;
     bool old_can_write, new_can_write;
 
     assert(reopen_state != NULL);
@@ -3314,6 +3318,13 @@ void bdrv_reopen_commit(BDRVReopenState *reopen_state)
     bs->open_flags         = reopen_state->flags;
     bs->read_only = !(reopen_state->flags & BDRV_O_RDWR);
 
+    /* Remove child references from bs->options and bs->explicit_options.
+     * Child options were already removed in bdrv_reopen_queue_child() */
+    QLIST_FOREACH(child, &bs->children, next) {
+        qdict_del(bs->explicit_options, child->name);
+        qdict_del(bs->options, child->name);
+    }
+
     bdrv_refresh_limits(bs, NULL);
 
     bdrv_set_perm(reopen_state->bs, reopen_state->perm,