blkdebug: use BlockDriverState's AioContext

Drop the assumption that we're using the main AioContext.  Convert
qemu_bh_new() to aio_bh_new() so we use the BlockDriverState's
AioContext.

The .bdrv_detach_aio_context() and .bdrv_attach_aio_context() interfaces
are not needed since no fd handlers, timers, or BHs stay registered when
requests have been drained.

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/blkdebug.c

@@ -471,7 +471,7 @@ static BlockDriverAIOCB *inject_error(BlockDriverState *bs,
     acb = qemu_aio_get(&blkdebug_aiocb_info, bs, cb, opaque);
     acb->ret = -error;
 
-    bh = qemu_bh_new(error_callback_bh, acb);
+    bh = aio_bh_new(bdrv_get_aio_context(bs), error_callback_bh, acb);
     acb->bh = bh;
     qemu_bh_schedule(bh);