Report backup's size when backup is created at backend

Most clouds provide end user a convenient way to protect their data by
creating a backup for their volume. User can create any backup copy at
anytime, And the cloud providers charge them by the backup size in total.
As the basic volume service, Cinder supports creating, restoring and
deleting backups plus the backup quotas, but our backup size isn't correct
at present, the backup's size always equals to the original volume's size
even if the incremental backup doesn't have any content in it.