Hi,
New features developed for the kernel are done in a separate git trees. When a feature is ready for inclusion into the main kernel tree, a pull request is sent. That workflow maintains a complete change history for the feature. Take performance events for example: you can go into Linus' git tree and see the complete history of changes. There's no reason the same methodology cannot be done for qemu.
It is done for qemu, pci and block are maintained that way for example. The key difference is that the patches which are accepted into the subsystem branches and then are pulled go through a full review @ qemu-devel before.
cheers, Gerd
