qtest_qmp_device_del() still uses the qmp("") hack to receive a
message, even though we have qmp_receive() since commit 66e0c7b187e.
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: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 tests/libqtest.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/libqtest.c b/tests/libqtest.c
index 9cb4096639..071d7eb7b1 100644
--- a/tests/libqtest.c
+++ b/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"));
 
-- 
2.17.1