Implement .bdrv_detach/attach_aio_context() interfaces to propagate
detach/attach to BDRVVmdkState->extents[].file.  The block layer takes
care of ->file and ->backing_hd but doesn't know about our extents
BlockDriverStates, which is also part of the graph.
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>