bhyve: add basic E820 implementation

There are some use cases where bhyve has to prepare some special memory
regions. E.g. GPU passthrough for Intel integrated graphic devices needs
to reserve some memory for the graphic device. So, bhyve has to inform
the guest about those memory regions. This information can be passed by
the qemu fwcfg interface. As qemu creates an E820 table, we can reuse
the existing fwcfg item "etc/e820".

This commit is the first one of a series. It only adds a basic
implementation for the creation of the E820 table. Some subsequent
commits will add more items to the E820 table and register it as fwcfg
item.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39545

(cherry picked from commit 9180daa1e34577aaccf3ff64cc63a5179c4f09d8)