net: Drop the NetLegacy structure, always use Netdev instead

net: Drop the NetLegacy structure, always use Netdev instead

Now that the "name" parameter is gone, there is hardly any difference
between NetLegacy and Netdev anymore, so we can drop NetLegacy and always
use Netdev to simplify the code quite a bit.

The only two differences that were really left between Netdev and NetLegacy:

1) NetLegacy does not allow a "hubport" type. We can continue to block
   this with a simple check in net_client_init1() for this type.

2) The "id" parameter was optional in NetLegacy (and an internal id
   was chosen via assign_name() during initialization), but it is mandatory
   for Netdev. To avoid that the visitor code bails out here, we have to
   add an internal id to the QemuOpts already earlier now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>