Xen stability: use monotonic time, check for page aligned I/O and add assert stubs

• [xen] Assert that pages passed to the grant share API are page-aligned.
  This always happens if they are created via Io_page.create, and
  probably not true if made by Cstruct.create.
• [xen] Use monotonic time for timing events, not wall-clock time.
• [xen] Provide functions that C code often uses for asserts (abort,
  printf, etc).