qapi schema: add Netdev types

NetdevTapOptions::sndbuf and NetdevDumpOptions::len use the new "size"
type.

v1->v2:
- NetLegacy::name is optional
- NetLegacyNicOptions::vectors is of type uint32
- NetdevVdeOptions::port and ::mode are of type uint16
- NetLegacy::vlan has type int32

v2->v3:
- NetLegacy::id is allowed and takes precedence over NetLegacy::name
- replace "@traits" with "@opts" in NetLegacy & Netdev descriptions

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