commit 3feb9059c31a5c08ec2c2ef81de439b6dec4f101
Author: Jim Fehlig <jfehlig@suse.com>
Date:   Thu Apr 28 11:30:19 2016 -0600

    libxl: don't add cache mode for qdisk cdrom drives
    
    qemu commit 91a097e7 forbids specifying cache mode for empty
    drives. Attempting to create a domain with an empty qdisk cdrom
    drive results in
    
    qemu-system-x86_64: -drive if=ide,index=1,readonly=on,media=cdrom,
       cache=writeback,id=ide-832: Must specify either driver or file
    
    libxl only allows an empty 'target=' for cdroms. By default, cdroms
    are readonly (see the 'access' parameter in xl-disk-configuration.txt)
    and forced to readonly by any tools (e.g. xl) using libxlutil's
    xlu_disk_parse() function. With cdroms always marked readonly,
    explicitly specifying the cache mode for cdrom drives can be dropped.
    The drive's 'readonly=on' option can also be set unconditionally.
    
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>

Index: xen-4.7.0-testing/tools/libxl/libxl_dm.c
===================================================================
--- xen-4.7.0-testing.orig/tools/libxl/libxl_dm.c
+++ xen-4.7.0-testing/tools/libxl/libxl_dm.c
@@ -1368,8 +1368,8 @@ static int libxl__build_device_model_arg
 
             if (disks[i].is_cdrom) {
                 drive = libxl__sprintf(gc,
-                         "if=ide,index=%d,readonly=%s,media=cdrom,cache=writeback,id=ide-%i",
-                         disk, disks[i].readwrite ? "off" : "on", dev_number);
+                         "if=ide,index=%d,readonly=on,media=cdrom,id=ide-%i",
+                         disk, dev_number);
 
                 if (target_path)
                     drive = libxl__sprintf(gc, "%s,file=%s,format=%s",