vhost-user-fs is a device and it should be possible to enable/disable
it with --without-default-devices, not --without-default-features.
Compute its default value in Kconfig to obtain the more intuitive
behavior.

In this case the configure options were undocumented, too.

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