Allow a mock object to receive synced_folders in config validation spec.

When using qemu session, the config is checked
if there are 9p synced folders.
We do not care about synced folders when we want to check
if the MAC address is defined correctly.

The result is a failure from the mock object:
`#<Double "vm"> received unexpected message :synced_folders with (no args)`

spec/unit/config_spec.rb

@@ -773,6 +773,8 @@ describe VagrantPlugins::ProviderLibvirt::Config do
       before do
         machine.config.instance_variable_get("@keys")[:vm] = vm
         allow(vm).to receive(:box).and_return(box)
+
+        allow(vm).to receive(:synced_folders).and_return({})
       end
 
       it 'is valid with valid mac' do