vmm: Support INOUT manual decode.

The inout instruction in AMD SVM requires DecodeAssist feature to decode the
segment override prefix. However, without that feature, we are still
able to decode by fetching the instruction directly.

Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51256