Setting "Paravirtualization Interface" in Vagrantfile

Found it. VBoxManage (the VirtualBox CLI tool) has an optional argument called --paravirtprovider. You can add that to the vb.customize call:

Vagrant.configure(2) do |config|
  config.vm.box = "ubuntu/trusty64"
  config.vm.provider "virtualbox" do |vb|
    vb.customize [
      "modifyvm", :id,
      "--memory", "1024",
      "--paravirtprovider", "kvm", # for linux guest
      "--cpus", "2"
    ]
  end
end

The other CPU settings are also available that way, vb.customize accepts the same argument as VBoxManage. Refer to VboxManage --help to get a list of all the options.