qemu 1.6.0: add vde networking support

The VDE (Virtual Distributed Ethernet) networking model is
useful for QEMU, as it allows muliple QEMU instances to connect
to virtual networks. More interesting is that a VDE server such
as vde_switch can connect a VDE network to the host machines
network, letting all QEMU instances on the virtual network
connect over the host's network.

This change adds support for VDE networking in QEMU by
using the vde project.