vhost: fix cleanup on not fully initialized device

If vhost_dev_init() failed, caller may still call vhost_dev_cleanup()
later. However, vhost_dev_cleanup() tries to remove the device from the
list even if it wasn't yet added, which may lead to crashes. Similarly
for the memory listener.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
