block: Don't poll in bdrv_set_aio_context()

The explicit aio_poll() call in bdrv_set_aio_context() was added in
commit c2b6428d as a workaround for bdrv_drain() failing to achieve
to actually quiesce everything (specifically the NBD client code to
switch AioContext).

Now that the NBD client has been fixed to complete this operation during
bdrv_drain(), we don't need the workaround any more.

It was wrong anyway: aio_poll() must always be run in the home thread of
the AioContext.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

block.c

@@ -5273,10 +5273,6 @@ void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context)
     bdrv_parent_drained_begin(bs, NULL, false);
     bdrv_drain(bs); /* ensure there are no in-flight requests */
 
-    while (aio_poll(ctx, false)) {
-        /* wait for all bottom halves to execute */
-    }
-
     bdrv_detach_aio_context(bs);
 
     /* This function executes in the old AioContext so acquire the new one in