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

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

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.

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

(cherry picked from commit 4e81f1caa7ff77f7fd31bd31f84b1a0dcfc8184e)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>