xen-blkfront: fix places not updated after introducing 64KB page granularity

Two places didn't get updated when 64KB page granularity was introduced,
this patch fix them.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>