bhyve: pass E820 table to guest

E820 table will be used to report valid RAM ranges and reserve special
memory areas like graphics memory for GPU passthrough.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39550

(cherry picked from commit 16f23f75432915e64a34357e7d5f26bca82f8219)