With aio=native (qemu-img bench -n) one or more requests can be completed
when a new request is submitted.  This in turn can cause bench_cb to
recurse before b->in_flight is updated.  This causes multiple I/Os
to be submitted with the same offset and, furthermore, the blk_aio_*
coroutines are never freed and qemu-img aborts.

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