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>