virtio serial doesn't work with virtio nic

If virtio NIC is not used virtserialport works and delivers data written to /dev/vport0p1 to localhost:4444:

qemu-system-x86_64 -enable-kvm -kernel mykernel -initrd myramdisk -device virtio-serial -chardev socket,host=localhost,port=4444,id=agent -device virtserialport,chardev=agent,name=hera.agent -net user -net nic

If using virtio nic, write to /dev/vport0p1 succeeds, but no data is delivered to localhost:4444:

qemu-system-x86_64 -enable-kvm -kernel mykernel -initrd myramdisk -device virtio-serial -chardev socket,host=localhost,port=4444,id=agent -device virtserialport,chardev=agent,name=hera.agent -net user -net nic,model=virtio

This bug exists in 2.0 release, Debian's QEMU 1.7 package and b3706faf from git.