bhyve: add basl support for length fields

ACPI tables have different layouts. So, there's no common position for
the length field. When tables are build by basl, the length is unknown
at the beginning. It has to be set after building the table.

Reviewed by: jhb
Approved by: manu (mentor)
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36989