tests/qtest: misc tweaks to readconfig

tests/qtest: misc tweaks to readconfig

The property name parameter is ignored when visiting a top
level type, but the obvious typo should be fixed to avoid
confusion. A few indentation issues were tidied up. We
can break out of the loop when finding the RNG device.
Finally, close the temp FD immediately when no longer
needed.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220809093854.168438-1-berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>