handbook/xen: minor fixes

[royger]

Summary

Setting vm.max_wired using sysrc doesn't work:

sysrc -f /etc/sysctl.conf vm.max_wired=-1

sysrc: vm.max_wired: name contains characters not allowed in shell

So switch to using echo and appending to file.

Fix runes for automatic bridge creation, use the recommended ones from
the bridging handbook chapter.

Also add a note that using a FreeBSD/Xen Dom0 requires booting in
legacy (BIOS) mode.

Finally remove the mention to xen.4th for the loader menu. With the
change to the Lua loader the custom Xen menu doesn't work anymore and
needs to be reworked.

[bcr]

Nice, thanks for those changes. I can commit them later today.

[royger]

Thank you very much for reviewing them! No rush to commit.