This is the simple approach to making sure that disk writes actually
hit disk before we tell the guest OS that IO has completed.  Thanks
to DMA_MULTI_THREAD the performance still seems to be adequate.

A fancier solution would be to make the sync/non-sync behaviour of
the qemu disk backing store tunable from the guest OS, by tuning
the IDE disk write cache on/off with hdparm, and having hw/ide.c
call ->fsync functions in the block backends.

I'm willing to code up the fancy solution if people prefer that.

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan