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

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>