libxl/qemu-xen: use cache=writeback for IDE and SCSI

Change caching mode from writethrough to writeback for upstream QEMU.

After a lengthy discussion, we came up with the conclusion that
WRITEBACK is OK for IDE.
See: http://marc.info/?l=xen-devel&m=133311527009773

Given that the same reasons apply to SCSI as well, change to writeback
for SCSI too.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

xen-unstable changeset: 25989:0807d1f7773d
Backport-requested-by: Ian Campbell <Ian.Campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_dm.c

@@ -549,10 +549,10 @@ static char ** libxl__build_device_model_args_new(libxl__gc *gc,
             if (disks[i].is_cdrom) {
                 if (disks[i].format == LIBXL_DISK_FORMAT_EMPTY)
                     drive = libxl__sprintf
-                        (gc, "if=ide,index=%d,media=cdrom", disk);
+                        (gc, "if=ide,index=%d,media=cdrom,cache=writeback", disk);
                 else
                     drive = libxl__sprintf
-                        (gc, "file=%s,if=ide,index=%d,media=cdrom,format=%s",
+                        (gc, "file=%s,if=ide,index=%d,media=cdrom,format=%s,cache=writeback",
                          disks[i].pdev_path, disk, format);
             } else {
                 if (disks[i].format == LIBXL_DISK_FORMAT_EMPTY) {
@@ -575,11 +575,11 @@ static char ** libxl__build_device_model_args_new(libxl__gc *gc,
                  */
                 if (strncmp(disks[i].vdev, "sd", 2) == 0)
                     drive = libxl__sprintf
-                        (gc, "file=%s,if=scsi,bus=0,unit=%d,format=%s",
+                        (gc, "file=%s,if=scsi,bus=0,unit=%d,format=%s,cache=writeback",
                          disks[i].pdev_path, disk, format);
                 else if (disk < 4)
                     drive = libxl__sprintf
-                        (gc, "file=%s,if=ide,index=%d,media=disk,format=%s",
+                        (gc, "file=%s,if=ide,index=%d,media=disk,format=%s,cache=writeback",
                          disks[i].pdev_path, disk, format);
                 else
                     continue; /* Do not emulate this disk */