hw/block/nvme: consider metadata read aio return value in compare

Currently in compare command metadata aio read blk_aio_preadv return
value ignored. Consider it and complete the block accounting.

Signed-off-by: Gollu Appalanaidu <anaidu.gollu@samsung.com>
Fixes: 0a384f92 ("hw/block/nvme: add compare command")
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>