tests/qtest: device-plug-test: Reverse the usage of double/single quotes

The usage of double/single quotes in test_pci_unplug_json_request()
should be reversed to work on both win32 and non-win32 platforms:

- The value of -device parameter needs to be surrounded by "" as
  Windows does not drop '' when passing it to QEMU which causes
  QEMU command line option parser failure.
- The JSON key/value pairs need to be surrounded by '' to make the
  JSON parser happy on Windows.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220824094029.1634519-36-bmeng.cn@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qtest/device-plug-test.c

@@ -95,7 +95,7 @@ static void test_pci_unplug_json_request(void)
     }
 
     QTestState *qtest = qtest_initf(
-        "%s -device '{\"driver\": \"virtio-mouse-pci\", \"id\": \"dev0\"}'",
+        "%s -device \"{'driver': 'virtio-mouse-pci', 'id': 'dev0'}\"",
         machine_addition);
 
     /*