aio-epoll: Fix use-after-free of node

aio_epoll_update needs the fields in node, so delay the free.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1447655534-13974-1-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
