pci: define a constant to represent a unmapped bar and use it. define a constant to represent a unmapped bar instead of -1 and use it. Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>