block: Deprecate "backing": ""

We have a clear replacement, so let's deprecate it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20180224154033.29559-8-mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>