annual HOWTO rampage, mini edition

[Greg Troxel <gdt%lexort.com@localhost>]

For several years I have updated the xen howto around end of year.
This year, the changes are smaller.

  https://wiki.netbsd.org/ports/xen/howto/

The big thing is that when updating to 4.18, if you have pv i386 guests,
you must flip type to pvh and set pvshim=1.


Things that could be improved in the HOWTO, if someone knows the answer:

  Is PVH support from a NetBSD 10 dom0 really experimental?  Are there
  any open bugs?  Note that we are recommending pvh/pvshim as the
  approach to run i386.

  If PVH works, why would you want to run PVHVM?  (I posit that there
  aren't good reasons.)

  If PVH works, why would you want to run HVM? (I posit that there
  aren't good reasons.)

  Does NetBSD 10 work as a dom0 in PVH mode?  If not, does current?

  How do you tell the system to use PVH for dom0?  Changing /boot.cfg?
  Just boot GENERIC instead of XEN3_DOM0?
  
  Does anyone have benchmarks for important workloads like building
  packages :-) in a domU, for PV vs PVH vs HVM vs PVHVM?