net: Flush queued packets when guest resumes

Since commit 6e99c631 "net/socket: Drop net_socket_can_send" and friends,
net queues need to be explicitly flushed after qemu_can_send_packet()
returns false, because the netdev side will disable the polling of fd.

This fixes the case of "cont" after "stop" (or migration).

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1436232067-29144-1-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>