virtio-blk: store opt_io_size with correct size

virtio-blk: store opt_io_size with correct size

The width of opt_io_size in virtio_blk_config is 32bit.  However, it's
written with virtio_stw_p; this may result in value truncation, and on
big-endian systems with legacy virtio in completely bogus readings in
the guest.

Use the appropriate accessor to store it.

Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200528225516.1676602-2-rvkagan@yandex-team.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>