xen/blkfront: Make sure that the device is fully ready before allowing release.

[ linux-2.6.18-xen changeset c1c57fea77e9 ]

Signed-off-by: Wim Colgate <wim@xensource.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>