Before we had 2 layes of calls, one in the virtio ring layer and
another in the virtio-blk layer. There wasn't any reall need for that.
The change allows the virtio-vring code to call a callback that the virtio-blk
registers and execute it directly.

At the moment it's hard coded as queue '0' but further on it should be a function
of the device requirements. It is also possible to move it towards the device function
since this thread doesn't do anything other than wake up the driver worker.

While doing it fix an off by 1 of i

Now basic data exchange seems to work (w/o interrupts yet)

We need only to store the virtual address returned from malloc.
virt_to_phys is the function translating the virtual mem into physical.

In addition, I founf a design error in the virtio PFN code. It will only
affect us on > 44 bit ram..