bhyve: add E820 dump function

For debugging purposes it is helpful to dump the E820 table.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39549

(cherry picked from commit a8a8e9af574c63fbecf4719e3bf184795dc98c51)