bhyve: add helper to fill a ACPI_GENERIC_ADDRESS

Reviewed by: jhb
Approved by: manu (mentor)
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D37407

(cherry picked from commit 60277ad75ec905d25182368d1022f56f43b490d5)