acpi nvdimm: fix wrong buffer size returned by DSM method

Currently, 'RLEN' is the totally buffer size written by QEMU and it is
ACPI internally used only. The buffer size returned to guest should
not include 'RLEN' itself

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>
