bhyve: add basl support for int values

In upcoming commits, bhyve will build some ACPI tables by it's own.
Therefore, it should be capable of appending int values to ACPI tables.

Reviewed by: jhb, markj (older version)
Approved by: manu (mentor)
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36987