Sorry for the somewhat redundant cover letter, but needed to note that:
This applies on top of "[PATCH v2 0/8] qemu-ga: add support for Windows", and
can also be obtained from:
git://github.com/mdroth/qemu.git qga-guest-sync-delimited
As noted in the commit there's a wiki write-up with more details on what
exactly this is for:
http://wiki.qemu.org/Features/QAPI/GuestAgent#QEMU_Guest_Agent_Protocol
It's not absolutely required, but it does make dealing with some communication
corner cases a heck of a lot easier.