xen: switch to use headers in contrib

These headers originate with the Xen project and shouldn't be mixed with
the main portion of the FreeBSD kernel. Notably they shouldn't be the
target of clean-up commits.

Switch to use the headers in sys/contrib/xen.

Reviewed by: royger

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

Merge in support for Xen HVM on amd64 architecture.