aio: return "AIO in progress" state from qemu_aio_wait

The definition of when qemu_aio_flush should loop is much simpler
than it looks.  It just has to call qemu_aio_wait until it makes
no progress and all flush callbacks return false.  qemu_aio_wait
is the logical place to tell the caller about this.

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