hw/nvme: reimplement dsm to allow cancellation

Prior to this patch, a loop was used to issue multiple "fire and forget"
aios for each range in the command. Without a reference to the aiocb
returned from the blk_aio_pdiscard calls, the aios cannot be canceled.

Fix this by processing the ranges one after another.

As a bonus, this fixes how metadata is cleared (i.e. we only zero it out
if the data was succesfully discarded).

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