vhost-scsi and vhost-user-scsi are two devices of their own; it should
be possible to enable/disable them with --without-default-devices, not
--without-default-features.  Compute their default value in Kconfig to
obtain the more intuitive behavior.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>