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

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

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

Cc: bhelgaas@google.com
Cc: linux-pci@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
OraBug: 25497392

(cherry picked from commit 58faf07b76817782ea20c392639569ea613cd439)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>