esx_vi_generator: Simplify generate_helper_source

The generate_helper_source() function returns a formatted string.
This could be achieved without the use of a local variable "source"
and string concatenation.
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NRadostin Stoyanov <rstoyanov1@gmail.com>

src/esx/esx_vi_generator.py

@@ -1092,26 +1092,19 @@ class ManagedObject(GenericObject):
 
 
     def generate_helper_source(self):
-        source = ""
-
         # lookup
-        source += "/* esxVI_Lookup%s */\n" % self.name
-        source += "ESX_VI__TEMPLATE__LOOKUP(%s,\n" % self.name
-        source += "{\n"
-
-        source += self.generate_lookup_code1()
-
-        source += "},\n"
-        source += "{\n"
-
-        source += self.generate_lookup_code2()
-
-        source += "})\n\n"
-
-        source += "\n\n"
-
-        return source
-
+        return (
+            "/* esxVI_Lookup%(name)s */\n"
+            "ESX_VI__TEMPLATE__LOOKUP(%(name)s,\n"
+            "{\n"
+            "%(lookup_code1)s},\n"
+            "{\n"
+            "%(lookup_code2)s})"
+            "\n\n\n\n"
+            % {"name": self.name,
+               "lookup_code1": self.generate_lookup_code1(),
+               "lookup_code2": self.generate_lookup_code2()}
+        )
 
 
 class Enum(Type):