Add vmgenc(4) driver for ACPI VM generation counter

The VM generation counter is a 128-bit value exposed by the BIOS via ACPI.
The value changes to another unique identifier whenever a VM is duplicated.
Additionally, ACPI provides notification events when such events occur.

The driver decodes the pointer to the UUID, exports the value to userspace
via OPAQUE sysctl blob, and forwards the ACPI notifications in the form of
an EVENTHANDLER invocation as well as userspace devctl events.

See design paper: https://go.microsoft.com/fwlink/p/?LinkID=260709