Add xenhvm.ko, which can be loaded along with a !XENHVM kernel in order to
provide support for the Xen/HVM environment.

This code is compiled with XENHVM defined; since this would result in the
(no longer used) "last processed" values being included in PCPU data
structures, an additional MODXENHVM define is used to exclude those.  This
allows KBI to be retained for both GENERIC and XENHVM kernel configurations
(which are not KBI compatible with each other).

This is a direct commit to stable/9, since stable/10 and HEAD have XENHVM
merged into the GENERIC kernel configuration (but the changes in stable/10
and HEAD cannot be MFCed as-is).

Discussed with: royger, gjb
Relnotes:	FreeBSD 9.3-RELEASE can run in Xen/HVM environments,
		including Amazon EC2, using GENERIC + xenhvm.ko.