libqtest: Clean up how we read device_del messages

qtest_qmp_device_del() still uses the qmp("") hack to receive a
message, even though we have qmp_receive() since commit 66e0c7b1.
Put it to use.

Bonus: gets rid of empty format strings.  A step towards compile-time
format string checking without triggering -Wformat-zero-length.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-3-armbru@redhat.com>

tests/libqtest.c

@@ -1090,7 +1090,7 @@ void qtest_qmp_device_del(const char *id)
     g_assert(response1);
     g_assert(!qdict_haskey(response1, "error"));
 
-    response2 = qmp("");
+    response2 = qmp_receive();
     g_assert(response2);
     g_assert(!qdict_haskey(response2, "error"));