Handle case where we're not xen

Previously if neither /sur/sbin/xl nor xm existed, then the `find_command`
method returned the array of XEN_COMMANDS. Since that wasn't nil, the resolver
tried to execute the Array of commands and failed (for the wrong reason).

Now explicitly return nil in the case neither file exists.

lib/facter/resolvers/xen.rb

@@ -66,6 +66,8 @@ def find_command
           return XEN_TOOLSTACK if num_stacks > 1 && File.exist?(XEN_TOOLSTACK)
 
           XEN_COMMANDS.each { |command| return command if File.exist?(command) }
+
+          nil
         end
       end
     end

spec/facter/resolvers/xen_spec.rb

@@ -27,6 +27,18 @@
     xen_resolver.invalidate_cache
   end
 
+  context 'when not xen' do
+    let(:evtchn_file) { false }
+    let(:proc_xen_file) { false }
+    let(:xvda1_file) { false }
+
+    it 'returns' do
+      allow(File).to receive(:exist?).with('/usr/sbin/xm').and_return(false)
+
+      expect(xen_resolver.resolve(:vm)).to be_nil
+    end
+  end
+
   context 'when xen is privileged' do
     context 'when /dev/xen/evtchn exists' do
       let(:domains) { load_fixture('xen_domains').read }