Xen monotonic

This fixes the problem of compiling with the wrong CFLAGS for Mini-OS, by removing the C stubs and using the new monotonic time API from https://github.com/mirage/mirage-platform/pull/108.

It also adds a Travis test script that builds:

* Stubs only, for Unix.
* Tracing, for Unix.
* Tracing, for Unix and Xen.