bhyve: save softc of ACPI devices

This will be useful for writing device specific ACPI tables or DSDT
methods.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39322

(cherry picked from commit 158adced65f844682d26e1e64f247de94212f135)