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

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

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: dmitry.torokhov@gmail.com
Cc: linux-input@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 81362c6f159dcb59fadd60927aa00497d715ca80)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>