Fix MachineState mapper so IsRunning actions work

The IsRunning action checks if `env[:machine].state.id == :running` but
this check was never passing as the protobuf-washed version of machine
state was yielding a machine state w/ a string value like `"running"`.

Easy fix in the mapper!

plugins/commands/serve/mappers/machine.rb

@@ -116,7 +116,7 @@ module VagrantPlugins
 
         def converter(m)
           Vagrant::MachineState.new(
-            m.id, m.short_description, m.long_description
+            m.id.to_sym, m.short_description, m.long_description
           )
         end
       end

test/unit/plugins/commands/serve/mappers_test.rb

@@ -36,4 +36,15 @@ describe VagrantPlugins::CommandServe::Mappers do
       expect(output).to eq([false])
     end
   end
+
+  context "MachineState" do
+    it "yields an id that's a symbol, not a string" do
+      input = Hashicorp::Vagrant::Sdk::Args::Target::Machine::State.new(
+        id: "running",
+      )
+      output = subject.map(input, to: Vagrant::MachineState)
+
+      expect(output.id).to eq(:running)
+    end
+  end
 end