reader: Updated VDL schema to nest the device sets under each file.

This CL restructures the VDL schema and nests the device sets under each file.
The "Linked" property is removed, because the unlinked device can be modeled as
a separate device set with a single device in it.

Change-Id: I023da3c7236490efea7f08d9ec63f125cb3a354c
diff --git a/vdl/device-set.vdl b/vdl/device-set.vdl
index c77dc5a..f803bae 100644
--- a/vdl/device-set.vdl
+++ b/vdl/device-set.vdl
@@ -8,7 +8,6 @@
   DeviceId string
   Page int32
   Zoom int32
-  Linked bool
 }
 
 type DeviceSet struct {
diff --git a/vdl/file.vdl b/vdl/file.vdl
index 28ba384..4c8b556 100644
--- a/vdl/file.vdl
+++ b/vdl/file.vdl
@@ -14,4 +14,5 @@
   Title string
   Size int64
   Type string
+  DeviceSets map[string]DeviceSet
 }