bhyve: add helper func to write a dsdt entry

The guest will check the dsdt to detect acpi devices. Therefore, add a
helper function to create such a dsdt entry for an acpi device.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38329