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

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

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: wei.liu2@citrix.com
Cc: paul.durrant@citrix.com
Cc: netdev@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
OraBug: 25497392

(cherry picked from commit f95842e7a9f235ef3b7d6d4b70fee2244149f1e7)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Conflicts:
	drivers/net/xen-netback/xenbus.c