net/virtio-user: fix multi-process support

net/virtio-user: fix multi-process support

This patch fixes the multi-process support for virtio-user.
Currently virtio-user just provides some limited secondary
process supports. Only some basic operations can be done in
secondary process on virtio-user port, e.g. getting port stats.
Actions which will trigger the communication with vhost backend
can't be done in secondary process for now, as the fds are
not synced between processes. The processing of server mode
devargs is also moved into virtio_user_dev_init().

Fixes: cdb068f031c6 ("bus/vdev: scan by multi-process channel")
Fixes: ee27edbe0c10 ("drivers/net: share vdev data to secondary process")
Cc: stable@dpdk.org
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>