kvm: Improve network syntax and tests

The current syntax when defining a NIC uses a pair of arguments, like this:

-net tap,vlan=0 -net nic,vlan=0,model=virtio

When using this pair, internally KVM will create a VLANState structure to send
ethernet frames between the guest NIC and the host tap device. This is
suboptimal and is going to be removed in the future. Plus, this combination
breaks vhost-net support.

The new syntax does not need the vlan craziness anymore, creating a direct 1:1
relationship between the backend device and the frontend device.

-netdev tap,id=netdev0 -device virtio-net-pci,mac=52:54:00:56:6c:55,netdev=netdev0

The current code does not verify if vhost_net is available before starting an
instance. All this tests are done verifying the KVM version installed on the
node.

Signed-off-by: Miguel Di Ciurcio Filho <miguel.filho@gmail.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>