Add legacy debug/test interfaces for kvm unit tests.

Implement the legacy debug/test interfaces expected by KVM-unit-tests'
realmode, emulator, and ioapic tests.

Submitted by: adam_fenn.io
Reviewed by: markj, grehan
Approved by: grehan (bhyve)
MFC after: 3 weeks
Relnotes: Yes
Differential Revision: https://reviews.freebsd.org/D27130