qapi: Generate FOO_str() macro for QAPI enum FOO

The next commit will put it to use.  May look pointless now, but we're
going to change the FOO_lookup's type, and then it'll help.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503564371-26090-13-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

scripts/qapi.py

@@ -1894,6 +1894,9 @@ typedef enum %(c_name)s {
 
     ret += mcgen('''
 
+#define %(c_name)s_str(val) \\
+    qapi_enum_lookup(%(c_name)s_lookup, (val))
+
 extern const char *const %(c_name)s_lookup[];
 ''',
                  c_name=c_name(name))