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>

include/qapi/util.h

@@ -11,6 +11,7 @@
 #ifndef QAPI_UTIL_H
 #define QAPI_UTIL_H
 
+const char *qapi_enum_lookup(const char *const lookup[], int val);
 int qapi_enum_parse(const char * const lookup[], const char *buf,
                     int def, Error **errp);