bhyve: Gracefully handle virtio-scsi with no conf

Fixes segfault with the command bhyve -s 0,virtio-scsi, which is used
by some third party software to probe bhyve for virtio-scsi support.

Reviewed by: jhb
MFC after: 1 day
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D29926