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.

and process pops requests from the ring.

In addition, implement the vfs device callbacks and wire them into virtio-blk
Need to test the later.

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.

this avoids a clash with the C++ std::list type.

This has no functional benefit, it just allows testing the async
block layer and can be reverted later.

Struct bio gains a condition variable and a mutex to allow waiting for
I/O completions.  Still no support for returning errors, but that can
be added fairly easily.

A console driver that wraps another with a spinlock.  We usually don't want
spinlocks, but if we want to debug the scheduler, we have to use them here.

While it works, it needs to move to the virtio layer

several instances

It was trapped when I run qemu w/ additional virtio-blk device.
It seems that multiple drivers of the same instance aren't working well

afterwards issue a matching number of read request to verify the content.

While doing it fix an off by 1 of i

requests and wait for the answers.

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

I've also added an osv/ prefix to make sure we don't clash with system headers.

We probably should either adopt this globally or drop it, but I fear we'll run into
problems with the latter approach.

It is illegal to put a field after a variable-length array, what's its offset?
Breaks mode=release.

This is inspired by the FreeBSD code, but nothing but the header and function
names are left of the original version.

Doesn't actually support real asynchronous execution as we're missing the concept
of waitqueues in osv as of now, but that will be added soon.

Created abstract base classes for device and drivers, changed the flow of driver loading / device probing