virtio: Fix compilation without CONFIG_VHOST_SCSI

Signed-off-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
bsd-user: Track change in FreeBSD SYSCTL(9) types

Originally from Garrett Cooper in FreeBSD PR ports/155558
http://www.freebsd.org/cgi/query-pr.cgi?pr=155558

Signed-off-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>