virtio-input: free config list

Clear the list when finalizing. The list is created during realize with
virtio_input_idstr_config() and later by further calls to
virtio_input_init_config() and virtio_input_add_config().

This leak can be reproduced with device-introspect-test -p
/x86_64/device/introspect/concrete.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
