bhyve: Fix virtio-console legacy configuration parsing

virtio-console is currently missing .pe_legacy_config, which prevents any
portN configuration from being parsed, and therefore no sockets will be
created.

Reviewed by: khng
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35142

(cherry picked from commit 3cdfaefa4b88c928bfb62e7fb6c7f7742d81f31c)