add support for boot 'network' from a vagrant public_network
add support for boot 'network' from a vagrant public_network

when use a public_network in vagrant, this is how libvirt domain xml looks like (notice that the interface type is 'direct'):

    <interface type="direct">
      <mac address="08:00:27:00:00:01"/>
      <source dev="br-rpi" mode="bridge"/>
      <model type="virtio"/>
      <boot order="2"/> <!-- this boot xml node will be inserted with this commit -->
      <address type="pci" domain="0x0000" bus="0x00" slot="0x05" function="0x0"/>
    </interface>

vagrant-libvirt/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,electrofelix/vagrant-libvirt,electrofelix/vagrant-libvirt,electrofelix/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt