Add support for bridge interface type

- Example for configuration inside Vagrantfile:
  test_vm1.vm.network :public_network, :dev => "br0", :mode => 'bridge',
    :type => "bridge", :model_type => "e1000"
Fix destroy in vagrant 1.5.1+

Addresses issue pradels/vagrant-libvirt#166
load missed module for 9p
add module 9pnet_virtio
Support for adding additional disks through the Vagrantfile.

This patch lets you define multiple additional disks in your
Vagrantfile.