libqtest: Clean up how we read the QMP greeting

qtest_init() still uses the qtest_qmp_discard_response(s, "") hack to
receive the greeting, even though we have qtest_qmp_receive() since
commit 66e0c7b187.  Put it to use.

Bonus: gets rid of an empty format string.  A step towards
compile-time format string checking without triggering
-Wformat-zero-length.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-4-armbru@redhat.com>

tests/libqtest.c

@@ -249,9 +249,11 @@ QTestState *qtest_init_without_qmp_handshake(bool use_oob,
 QTestState *qtest_init(const char *extra_args)
 {
     QTestState *s = qtest_init_without_qmp_handshake(false, extra_args);
+    QDict *greeting;
 
     /* Read the QMP greeting and then do the handshake */
-    qtest_qmp_discard_response(s, "");
+    greeting = qtest_qmp_receive(s);
+    qobject_unref(greeting);
     qtest_qmp_discard_response(s, "{ 'execute': 'qmp_capabilities' }");
 
     return s;