auto br0
iface br0 inet manual
bridge-ports eth1
bridge-stp off
bridge-fd 0
bridge-vlan-aware yes
bridge-vlan 2-4000
<network>
<name>br0</name>
<uuid>76b8ef6e-342b-4a9d-8298-de5f12b448bc</uuid>
<forward mode='bridge'/>
<bridge name='br0'/>
</network>
qemu -netdev socket...