Allow usage of public/private networks as management networks

This requires that the qemu_use_agent is enabled and at least one
network is defined in the Vagrantfile that is either private or public.

The restriction that qemu_use_agent need be enabled may be removed
subsequently for networks using a static IP address. That should be
considered a subsequent problem.

Fixes: #1204
Fixes: #1124
Fixes: #1057

tests/qemu_agent/Vagrantfile

@@ -6,7 +6,9 @@
 Vagrant.configure("2") do |config|
   config.vm.box = "generic/debian10"
   config.vm.synced_folder ".", "/vagrant", disabled: true
+  config.vm.network "private_network", type: "dhcp"
   config.vm.provider :libvirt do |libvirt|
     libvirt.qemu_use_agent = true
+    libvirt.mgmt_attach = false
   end
 end