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

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

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

Cc: tomi.valkeinen@ti.com
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
OraBug: 25497392

(cherry picked from commit eaf46e181ec3cc3b6eafdbe8e30fb5a03ebbde68)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>