hw/nvme: reimplement flush to allow cancellation

Prior to this patch, a broadcast flush would result in submitting
multiple "fire and forget" aios (no reference saved to the aiocbs
returned from the blk_aio_flush calls).

Fix this by issuing the flushes one after another.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>