net/queue: introduce NetQueueDeliverFunc

net/queue.c has logic to send/queue/flush packets but a
qemu_deliver_packet_iov() call is hardcoded. Abstract this
func so that we can use our own deliver function in netfilter.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>