bhyve(8): Add VM Generation Counter ACPI device

Add an implementatation of the 'Virtual Machine Generation ID' spec to
Bhyve. The spec provides a randomly generated GUID (at bhyve start) in
device memory, along with an ACPI device with _CID VM_Gen_Counter and ADDR
evaluating to a Package pointing at that GUID.

A GPE is defined which Notifies the ACPI Device when the generation changes
(such as when a snapshot is rolled back). At this time, Bhyve does not
support snapshotting, so the GPE is never actually raised.

Suggested by: rpokala
Discussed with: grehan
Differential Revision: https://reviews.freebsd.org/D23165