xen/xen-blkback: preq.dev is used without initialized

If call xen_vbd_translate failed, the preq.dev will be not initialized.
Use blkif->vbd.pdevice instead (still better to print relative info).

Note that before commit 01c681d4
(xen/blkback: Don't trust the handle from the frontend.)
the value bogus, as it was the guest provided value from req->u.rw.handle
rather than the actual device.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Acked-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>