Add a new section about Xen to the virtualization chapter.
It is based on the entries in the FreeBSD Wiki andi Xen's
own instructions specific to FreeBSD.

In particular, it describes how to configure the host machine,
set up the Dom0, and add a DomU VM afterwards.

Reviewed by: royger, wblock
Differential Revision: https://reviews.freebsd.org/D10774