Bug fix: do not use QEMU sessions (implements #53)

As mentioned in #53, Vagrant 2.2 uses QEMU sessions on Fedora, breaking
the creation of custom virtual networks.

Fix: disable the use of QEMU sessions in Vagrantfile

netsim/templates/provider/libvirt/Vagrantfile.j2

@@ -6,6 +6,7 @@ Vagrant.configure("2") do |config|
 {%   if n.mgmt is defined and n.mgmt.mac is defined %}
     {{ name }}.vm.provider :libvirt do |domain|
       domain.management_network_mac = "{{n.mgmt.mac}}"
+      domain.qemu_use_session = false
     end
 {%   endif %}
 {%  include n.device ~ "-domain.j2" %}