vvfat: Fix qcow write target driver specification

First, bdrv_open_child() expects all options for the child to be
prefixed by the child's name (and a separating dot). Second,
bdrv_open_child() does not take ownership of the QDict passed to it but
only extracts all options for the child, so if a QDict is created for
the sole purpose of passing it to bdrv_open_child(), it needs to be
freed afterwards.

This patch makes vvfat adhere to both of these rules.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-id: 20160711135452.11304-1-mreitz@redhat.com
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/vvfat.c

@@ -3018,9 +3018,10 @@ static int enable_write_target(BlockDriverState *bs, Error **errp)
     }
 
     options = qdict_new();
-    qdict_put(options, "driver", qstring_from_str("qcow"));
+    qdict_put(options, "write-target.driver", qstring_from_str("qcow"));
     s->qcow = bdrv_open_child(s->qcow_filename, options, "write-target", bs,
                               &child_vvfat_qcow, false, errp);
+    QDECREF(options);
     if (!s->qcow) {
         ret = -EINVAL;
         goto err;