x-qemux86-directdisk.wks.in: Use dynamic rootfstype and remove rw

The default configuration for the rootfs is now read-only. The ro
argument is injected through APPEND in read-only-rootfs hooks so all we
need to do is to drop `rw`.

We also let rootfstype be defined dynamically so we can support multiple
configuration in the future.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>