um: virtio_uml: fix memory leak on init failures

If initialization fails, e.g. because the connection failed,
we leak the 'vu_dev'. Fix that. Reported by smatch.

Fixes: 5d38f324 ("um: drivers: Add virtio vhost-user driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>