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

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

The usage of double/single quotes in test_q35_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.

Fixes: a12f1a7e56b7 ("tests/x86: Add subtest with 'q35' machine type to device-plug-test")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20221028045736.679903-5-bin.meng@windriver.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>