Add serial number support for virtio_blk, V4a

This patch extracts the opaque data from pci i/o
region 0 via the added VIRTIO_BLK_F_IDENTIFY
field.  By convention this data takes the form of
that returned by an ATA IDENTIFY DEVICE command,
however the driver (except for structure size)
makes no interpretation of the data.  The structure
data is copied wholesale to userspace via a
HDIO_GET_IDENTITY ioctl command (eg: hdparm -i <dev>).

Signed-off-by: john cooper <john.cooper@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>