piix3_ide_xen_class_init is identical to piix3_ide_class_init
except it's buggy as it does not set exit and does not disable
hotplug properly.
Switch to the generic one.
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>