nvdimm acpi: abstract the operations for root & nvdimm devices

It separates the operations between root device and nvdimm devices
in order to introducing label functions support for nvdimm device

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