This isn't fully cooked yet, but pretty close.  The basic idea is to 
make the aio usage in block-raw go to a set of function pointers and 
allow multiple simultaneous AIO implementations.

I converted the posix-aio support to this, and also introduced a "unix" 
aio which just uses O_NONBLOCK and select().  The later only supports 1 
simultaneous request per-fd but currently posix-aio is limited to that 
too.  At least with my QEMU testing, the unix aio implementation 
outperforms posix-aio by a factor of 2.

And it uses no signals...

I'm inclined to suggest that we use signalfd with posix-aio, and for 
older guests, just fall back to unix aio.  We can also introduce a 
linux-aio and use that when possible.

Regards,

Anthony Liguori