Discover that bio->bio_offset is zero while the data starts in
the middle of the page so I adjusted the code accordingly.
Will need to sort it out later on.

Time might stop under some hosts without this fix.

The logic is called as callback invoked by virtio interrupts.

In addition, polish the test code. Before all of the write request
had filled the ring and left no space for the read requests. Now I rotate
between the two.

Only it's usage within the array should be within limits.

Add a function for available ring content

This makes it easier to create a thread with no special properties.

This makes it easy to configure a thread with various parameters.  The first
is the existing stack_info parameter.

pushing it into a dummy bio structure. In addition, expand the test some more

The apic needs to be programmed on each cpu that is brought up.

Each cpu has its own event timer; we rely on this.  Document the requirement.

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