Networking support

This adds preliminary networking support. You can configure multiple
network interfaces that are in either bridged mode or NAT mode.

To start a QEMU VM in bridged mode:

  go run launcher/launchvm.go \
    --name "debian" \
    --driver "qemu" \
    --image ~/Downloads/debian_wheezy_amd64_standard.qcow2 \
    --net bridge,virbr0

To start a QEMU VM in NAT networking:

  go run launcher/launchvm.go \
    --name "debian" \
    --driver "qemu" \
    --image ~/Downloads/debian_wheezy_amd64_standard.qcow2 \
    --net nat,tcp:2222:22,tcp:8080:80

where port 2222 on host is forwarded to post 22 on the guest and port
8080 on the host to port 80 on the guest.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>