bhyve: add allocation function to E820

This function makes it easy to allocate new E820 entries. It will be
used to allocate graphics memory for Intel integrated graphic devices.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39547

(cherry picked from commit 5597f564870e94d56111dec638b8859423c936a9)