bhyve: add common memory holes to E820 table

The VGA and the ROM memory ranges can't be used as system memory. For
that reason, remove them from the E820 table.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39546

(cherry picked from commit 059af92a300f76653048c042559f7d7d8fd8f99c)