Add Vchan support to Mirage config.ml

This adds a couple of `Vchan` implementations:

* `vchan_loopback` uses the `Vchan.In_memory` loopback implementation to provide a portable but non-remote default.  This works on (e.g.) MacOS X and provides a default vchan that should work anywhere.
* `vchan_xen` can be enabled in config.ml to use the real implementations.  This will only work on a host with Xen installed (using `Vchan_lwt_unix`) or on a Xen backend directly (using `Vchan_xen`).

I'm about to hook in the Conduit support next, so that name resolution across VMs will work using these exposed interfaces.