xen: xen-pciback: fix warning Using plain integer as NULL pointer

Changes passing function argument 0 to NULL to avoid below sparse
warning

 CHECK   drivers/xen/xen-pciback/xenbus.c
drivers/xen/xen-pciback/xenbus.c:700:51: warning: Using plain integer as
NULL pointer

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>