hw/ide: drop iov field from IDEBufferedRequest

@iov is used only to initialize @qiov. Let's use new
qemu_iovec_init_buf() instead, which simplifies the code.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190218140926.333779-17-vsementsov@virtuozzo.com
Message-Id: <20190218140926.333779-17-vsementsov@virtuozzo.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>