bhyve: check for errors when writing device specific DSDT entries

At the moment, this function can't fail. This behaviour will change in
the future. In preparation to that, convert the return type to int in
order to be able to check for errors.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39422