net/virtio-user: fix init when using existing tap

net/virtio-user: fix init when using existing tap

When attaching to an existing mono queue tap, the virtio-user was not
reporting that the virtio device was not properly initialised which
prevented from starting the port later.

$ ip tuntap add test mode tap
$ dpdk-testpmd --vdev \
  net_virtio_user0,iface=test,path=/dev/vhost-net,queues=2 -- -i

...
virtio_user_dev_init_mac(): (/dev/vhost-net) No valid MAC in devargs or
device, use random
vhost_kernel_open_tap(): TUNSETIFF failed: Invalid argument
vhost_kernel_enable_queue_pair(): fail to open tap for vhost kernel
virtio_user_start_device(): (/dev/vhost-net) Failed to start device
...
Configuring Port 0 (socket 0)
vhost_kernel_open_tap(): TUNSETIFF failed: Invalid argument
vhost_kernel_enable_queue_pair(): fail to open tap for vhost kernel
virtio_set_multiple_queues(): Multiqueue configured but send command
failed, this is too late now...
Fail to start port 0: Invalid argument
Please stop the ports first
Done

The virtio-user with vhost-kernel backend was going through a lot
of complications to initialise tap fds only when using them.

For each qp enabled for the first time, a tapfd was created via
TUNSETIFF with unneeded additional steps (see below) and then mapped to
the right qp in the vhost-net backend.
Unneeded steps (as long as it has been done once for the port):
- tap features were queried while this is a constant on a running
  system,
- the device name in DPDK was updated,
- the mac address of the tap was set,

On subsequent qps state change, the vhost-net backend fd mapping was
updated and the associated queue/tapfd were disabled/enabled via
TUNSETQUEUE.

Now, this patch simplifies the whole logic by keeping all tapfds opened
and in enabled state (from the tap point of view) at all time.

Unused ioctl defines are removed.

Tap features are validated earlier to fail initialisation asap.
Tap name discovery and mac address configuration are moved when
configuring qp 0.

To support attaching to mono queue tap, the virtio-user driver now tries
to attach in multi queue first, then fallbacks to mono queue.

Finally (but this is more for consistency), VIRTIO_NET_F_MQ feature is
exposed only if the underlying tap supports multi queue.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>