Update to r52813:

Update the Xen chapter to support newer versions of Xen.

This patch clarifies which versions of FreeBSD support Xen 4.7 (PVHv1)
and Xen 4.11 or greater (PVHv2). Additionally, improve a few of the
instructions. Use onifconsole for Xen, add a line to loader.conf to
load the tap device. Remove restrictions to use MMCFG PCI accesses for
Xen >= 4.11. Make /var/log/xen stand out a bit more by making it a tip.
Finally, the example guest was creating a 10.3 version of FreeBSD,
which is no longer supported. Update that to 12.0 and wrap it in
replaceable tags so that people know they can put something else there.