Implement Xen's vchan protocol

[thumphries]

Some years after Adam's glorious IVC mechanism debuted, a library appeared in Xen by the name of libvchan. It is very similar in functionality to our mechanism, but with a couple of bonuses:

• A nice C implementation for Dom0, letting us deprecate libIVC and get a robust replacement for free
• It appears to have some convention for closing the channel and ending grants, something I'd like to make use of
• Mirage has a nice implementation, so we can all get along

Seems nice to have.