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

(cherry picked from commit e22f5ce2bf4c1a53a1551ae855f42d582eeda20e)