vhost: allow vhost-scsi driver to be built-in

It's useful to allow vhost-scsi to be built-in when testing vhost in L1
+ L2 VMs and booting L1 VM with QEMU '-kernel' option.

Signed-off-by: Greg Edwards <gedwards@ddn.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>