qemu: add support for bridge networking

[rgl]

I'm trying to install ESXi 7.0 and the combination of using the vmxnet3 nic and the qemu user mode networking does not work (please note that the e1000e nic does not work either).

I've manually used the qemu bridge mode and it worked nicely, please see the raw qemu-kvm section at https://github.com/rgl/esxi-vagrant/blob/feature-upgrade-to-esxi-7/README.md.

Essentially, I would be fine to just add the net_bridge property, set it as "net_bridge": "virbr0" and require the user to already have virbr0 properly configured before calling packer.

[ghost]

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