vmgenid(4): Integrate as a random(4) source

[cem]

Summary

The number is currently published by the vmgenid(4) driver and has no
"entropy," but should be integrated quickly on VM rewind events to avoid
duplicate sequences.

Test Plan

(If we think the entropy would be more valuable, we could restrict
publication of the GUID from vmgenid(4). I do not know if it is available
to unprivileged users in any other way.)

[markm]

LGTM