xen/manage: correct return value check on xenbus_scanf()

A negative return value indicates an error; in fact the function at
present won't ever return zero.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>