Change Xen event channel "last processed" values from per-CPU to global.
In FreeBSD 9.x we only run this code on (virtual) CPU #0, so there is no
need for these to be per-CPU values.  Retain the values in the per-CPU
data structure (when compiled with options XENHVM) for KBI compatibility.

This is a direct commit to stable/9, since the relevant code has been
substantially changed (in ways which cannot be easily MFCed) in HEAD and
stable/10.

Submitted by:	royger (earlier version)