xen/xenbus: Fixed over 80 character limit issue

Fixed the format length of the xenbus_backend_ioctl()
function to meet the 80 character limit in
xenbus_dev_backend.c

Signed-off-by: Lisa Nguyen <lisa@xenapiadmin.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>