[qemu] hdparm tunable IDE write cache for HVM

[qemu] hdparm tunable IDE write cache for HVM
qemu 0.8.2 has a flush callback to the storage backends, so now it is
possible to implement hdparm tunable IDE write cache enable/disable for
guest domains, allowing people to pick speed or data consistency on a
case by case basis.

As an added benefit, really large LBA48 IOs will now no longer be broken
up into smaller IOs on the host side.

From: Rik van Riel <riel@redhat.com>
Signed-off-by: Christian Limpach <Christian.Limpach@xensource.com>