um: virtio_uml: Fix use-after-free after put_device in probe

When register_virtio_device() fails in virtio_uml_probe(),
the code sets vu_dev->registered = 1 even though
the device was not successfully registered.
This can lead to use-after-free or other issues.

Fixes: 04e5b1fb ("um: virtio: Remove device on disconnect")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>