nvdimm acpi: prebuild nvdimm devices for available slots

For each NVDIMM present or intended to be supported by platform,
platform firmware also exposes an ACPI Namespace Device under
the root device

So it builds nvdimm devices for all slots to support vNVDIMM hotplug

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
