Validate partial mocks (#1417)

Switch to using explicit references to objects to be partially mocked
and remove the need to resolve the string constants as this will catch
more instances of calls to invalid or missing methods.

Rework how the vm is added to the machine for one of the tests as it is
not a method and instead is provided via internal state being exposed
with a helper.

spec/unit/config_spec.rb

@@ -607,7 +607,9 @@ describe VagrantPlugins::ProviderLibvirt::Config do
 
     context 'with mac defined' do
       let (:vm) { double('vm') }
-      before { expect(machine.config).to receive(:vm).and_return(vm) }
+      before do
+        machine.config.instance_variable_get("@keys")[:vm] = vm
+      end
 
       it 'is valid with valid mac' do
         expect(vm).to receive(:networks).and_return([[:public, { mac: 'aa:bb:cc:dd:ee:ff' }]])