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

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

Use xenbus_read_unsigned() instead of xenbus_scanf() when possible.
This requires to change the type of some reads from int to unsigned,
but these cases have been wrong before: negative values are not allowed
for the modified cases.

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 f27dc1ac56865c2cc43d0ec3110a2b4a95b04e1d)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Conflicts:
	drivers/block/xen-blkfront.c