bgp: fix withdraw in EventPrefix notification

A lost path's is_withdraw is not True so we can't use it. Let's pass
path and is_withdraw information separately to emit_signal().

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Reviewed-by: Hiroshi Yokoi <yokoi.hiroshi@po.ntts.co.jp>