libqtest: Clean up how we read device_del messages

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 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>
Message-Id: <20180806065344.7103-3-armbru@redhat.com>