bhyve: Enable virtio-scsi legacy config parsing.

The previous commit added the handler to parse the command line
options for virtio-scsi devices but forgot to set the correct function
pointer to point to the handler.

Reported by: vangyzen
Reviewed by: vangyzen
Fixes: 621b5090487de9fed1b503769702a9a2a27cc7bb
Differential Revision: https://reviews.freebsd.org/D29438

(cherry picked from commit 4d5460a720c59a4404eb1df1b768d2f16b2f341a)