xen: make use of xenbus_read_unsigned() in xenbus

xen: make use of xenbus_read_unsigned() in xenbus

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.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
OraBug: 25497392

(cherry picked from commit 999c9af9e3a2535d9ad41182e93eb128e587eb84)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>