xentrace: Allow xentrace to handle >4G of trace data.

xentrace: Allow xentrace to handle >4G of trace data.
It was previously assert'ing when it hit 4G.

Also, because the trace buffer is not a power of 2 in size,
using modulo arithmetic to address the buffer does not work
when the index wraps around 2^32.

This patch fixes both issues, and as a side effect, removes all
integer division from the hypervisor side of the trace mechanism.

Signed-off-by: Michael A Fetterman <Michael.Fetterman@cl.cam.ac.uk>