use snapshot device in template to make it updatable with running appvms

vm_conf_files/appvm-template.conf

@@ -10,7 +10,7 @@ root="/dev/mapper/dmroot"
 memory = 400
 name = "%VMNAME%"
 
-disk = [ 'file:%TEMPLATEDIR%/root.img,xvda,r', 
+disk = [ 'snapshot:%TEMPLATEDIR%/root.img:%TEMPLATEDIR%/root-cow.img,xvda,r', 
 	 'file:%VMDIR%/private.img,xvdb,w',
 	 'file:%VMDIR%/root-cow.img,xvdc,w',
 	 'file:%VMDIR%/swap-cow.img,xvdd,w'

vm_conf_files/templatevm.conf

@@ -10,7 +10,7 @@ root="/dev/mapper/dmroot"
 memory = 400
 name = "%TEMPLATENAME%"
 
-disk = [ 'file:/var/lib/qubes/vm-templates/%TEMPLATENAME%/root.img,xvda,w', 
+disk = [ 'origin:/var/lib/qubes/vm-templates/%TEMPLATENAME%/root.img:/var/lib/qubes/vm-templates/%TEMPLATENAME%/root-cow.img,xvda,w', 
 	 'file:/var/lib/qubes/vm-templates/%TEMPLATENAME%/private.img,xvdb,w',
        ]