convert net_init_vde() to NetClientOptions

v1->v2:
- NetdevVdeOptions::port and ::mode are of type uint16. Remove superfluous
  range checks.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
