nvdimm acpi: clean up nvdimm_build_acpi

To make the code more clearer, we
1) check ram_slots first, and build ssdt & nfit only when it is available
2) use nvdimm_get_plugged_device_list() to check if there is nvdimm device
   plugged

Suggested-by: Igor Mammedov <imammedo@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>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
