I've basically got a choice of making libvirt always ad '-aio linux'
or never add it at all. My inclination is to the latter since it is
compatible with existing QEMU which has no -aio option. Presumably
'-aio linux' is intended to provide some performance benefit so it'd
be nice to use it. If we can't express some criteria under which it
should be turned on, I can't enable it; where as if you can express
some criteria, then QEMU should apply them automatically.
I'm of the view that '-aio auto' would be a really good option - and
when it's proven itself, it should be the default. It could work on
all QEMU hosts: it would pick synchronous IO when there is nothing else.