hw/nvme: reimplement the copy command to allow aio cancellation

Before this patch the code would issue several aios simultaneously
without saving a reference to the aiocb. Without the aiocb reference the
individual copies cannot be canceled.

Fix this by issuing copies of the ranges one after another.

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