bhyve: maintain a list of acpi devices

The list is used to generate the dsdt entry for every acpi device.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D3830

(cherry picked from commit fbd045021d9ffdae5e5bb20e1d1890fd4b46498e)