extend layout-function get_device_name so it can handle /dev/vg/lv forma...

hi:
   this patch tried to fix when drbd use "/dev/vg/lv" as backend device,  rear won't recognize it's the same device as "/dev/mapper/vg-lv". since there maybe other situation which also use "/dev/vg/lv" as backend device, maybe it's better to fix it in the layout-function and other script can benefit with it.