tests: New helper qtest_qmp_receive_success()

Commit b21373d071 copied wait_command() from tests/migration-test.c
to tests/tpm-util.c.  Replace both copies by new libqtest helper
qtest_qmp_receive_success().  Also use it to simplify
qtest_qmp_device_del().

Bonus: gets rid of a non-literal format string.  A step towards
compile-time format string checking without triggering
-Wformat-nonliteral.

Cc: Thomas Huth <thuth@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-17-armbru@redhat.com>

tests/libqtest.h

@@ -146,6 +146,23 @@ void qtest_qmp_eventwait(QTestState *s, const char *event);
  */
 QDict *qtest_qmp_eventwait_ref(QTestState *s, const char *event);
 
+/**
+ * qtest_qmp_receive_success:
+ * @s: #QTestState instance to operate on
+ * @event_cb: Event callback
+ * @opaque: Argument for @event_cb
+ *
+ * Poll QMP messages until a command success response is received.
+ * If @event_cb, call it for each event received, passing @opaque,
+ * the event's name and data.
+ * Return the success response's "return" member.
+ */
+QDict *qtest_qmp_receive_success(QTestState *s,
+                                 void (*event_cb)(void *opaque,
+                                                  const char *name,
+                                                  QDict *data),
+                                 void *opaque);
+
 /**
  * qtest_hmp:
  * @s: #QTestState instance to operate on.