net: make use of async packet sending API in tap client

If a packet is queued by qemu_send_packet(), remove I/O
handler for the tap fd until we get notification that the
packet has been sent.

A not insignificant side effect of this is we can now
drain the tap send queue in one go without fear of packets
being dropped.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>