test-char: destroy chardev-udp after test

this is only not a problem if the test is last in a suite,
otherwise it makes the following main_loop() calls to fail

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1499342940-56739-7-git-send-email-anton.nefedov@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>