xen: make use of xenbus_read_unsigned() in xen-blkback

xen: make use of xenbus_read_unsigned() in xen-blkback

Use xenbus_read_unsigned() instead of xenbus_scanf() when possible.
This requires to change the type of one read from int to unsigned,
but this case has been wrong before: negative values are not allowed
for the modified case.

Cc: konrad.wilk@oracle.com
Cc: roger.pau@citrix.com
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
OraBug: 25497392

(cherry picked from commit 8235777b2068e3280b6fa1413f1940ade31f0adf)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>