Xen event channel interface for MirageOS

Event channels are the Xen equivalent of interrupts, used to signal when data (or space) is available for processing. There are 2 distinct implementations:

1. a Xen shared-memory + hypercall protocol for kernelspace
2. a binding to a set of libxc functions which access /dev/xen/evtchn for userspace (see xen-evtchn-unix opam package).