bhyve: introduce acpi_device_emul struct

It'll be easier to add new properties to the ACPI device emulation if we
have a struct which holds all device specific properties. In some future
commits the acpi_device_emul struct will be expanded to include some
device specific functions to build ACPI tables.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39319