net/virtio_user: fix error management during init

net/virtio_user: fix error management during init

Currently, when virtio_user device fails to be started (e.g., vhost
unix socket does not exit), the init function does not return struct
rte_eth_dev (and some other structs) back to ether layer. And what's
more, it does not report the error to upper layer.

The fix is to free those structs and report error when failing to
start virtio_user devices.

Fixes: ce2eabdd43ec ("net/virtio-user: add virtual device")

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>