tests: Clean up string interpolation around qtest_qmp_device_add()

Leaving interpolation into JSON to qmp() is more robust than building
QMP input manually, as explained in the commit before previous.

qtest_qmp_device_add() and its wrappers interpolate into JSON as
follows:

* qtest_qmp_device_add() interpolates members into a JSON object.

* So do its wrappers qpci_plug_device_test() and usb_test_hotplug().

* usb_test_hotplug() additionally interpolates strings and numbers
  into JSON strings.

Clean them up:

* Have qtest_qmp_device_add() take its extra device properties as
  arguments for qdict_from_jsonf_nofail() instead of a string
  containing JSON members.

* Drop qpci_plug_device_test(), use qtest_qmp_device_add()
  directly.

* Change usb_test_hotplug() parameter @port to string, to avoid
  interpolation.  Interpolate @hcd_id separately.

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>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-15-armbru@redhat.com>

tests/virtio-blk-test.c

@@ -666,8 +666,9 @@ static void pci_hotplug(void)
     qs = pci_test_start();
 
     /* plug secondary disk */
-    qpci_plug_device_test("virtio-blk-pci", "drv1", PCI_SLOT_HP,
-                          "'drive': 'drive1'");
+    qtest_qmp_device_add("virtio-blk-pci", "drv1",
+                         "{'addr': %s, 'drive': 'drive1'}",
+                         stringify(PCI_SLOT_HP));
 
     dev = virtio_blk_pci_init(qs->pcibus, PCI_SLOT_HP);
     g_assert(dev);