vmgenc.4: Add VM Generation ID Counter manual

Document the vmgenc(4) ACPI driver which detects virtual machine
cloning and snapshot restoration via the VM Generation ID
specification. The driver reseeds the kernel entropy pool when
a generation change is detected.

MFC after: 3 days
Reviewed by: cem (previous), ziaee
Signed-off-by: Christos Longros <chris.longros@gmail.com>
Differential Revision: https://reviews.freebsd.org/D56011