stream: fix ratelimiting corner case

This fixes inability to make progress in streaming if the quota is set
to less than the amount of data that an I/O operation has to write.

In this case, limit->dispatched + n will always be above the quota and,
due to the "goto retry" to recheck cancellation and allocation, streaming
will livelock.

This can be reproduced with "block_job_set_speed ide0-hd0 1b".  Of course,
with this patch the requested limit will not be obeyed.  That could be
done with another patch that caps is_allocated's n argument by the slice
quota.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
