Fix PSImageReference format

Author: matthchr

src/Batch/Batch/Batch.format.ps1xml

@@ -62,6 +62,7 @@
             </TableColumnItems>
           </TableRowEntry>
         </TableRowEntries>
-      </TableControl>    </View>
+      </TableControl>
+    </View>
   </ViewDefinitions>
 </Configuration>

src/Batch/Batch/Models/PSImageReference.cs

@@ -0,0 +1,17 @@
+namespace Microsoft.Azure.Commands.Batch.Models
+{
+    public partial class PSImageReference
+    {
+        public override string ToString()
+        {
+            if (string.IsNullOrEmpty(VirtualMachineImageId))
+            {
+                return $"{Publisher}:{Offer}:{Sku}:{Version}";
+            }
+            else
+            {
+                return VirtualMachineImageId;
+            }
+        }
+    }
+}