handbook/virtualization: add note on workaround for Parallels

A user reported high CPU utilization on a fresh install of 14.0
on Parallels (MacOS, M1/Apple Silicon). Disabling the new ACPI
ged(4) device worked around the problem. Document this (for now).

Reviewed by: pauamma_gundo.com
Differential Revision: https://reviews.freebsd.org/D45103