virtio-net: adding all queues in .realize()

Instead of adding queues for multiqueue during feature set. This patch
did this in .realize(), this will help the following patches that
count the number of virtqueues used in .device_plugged() callback.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>