qapi: convert netdev_add

qapi: convert netdev_add

This is not a full QAPI conversion, but an intermediate step.

In essence, do_netdev_add() is split into three functions:

 1. netdev_add(): performs the actual work. This function is fully
    converted to Error (thus, it's "qapi-friendly")

 2. qmp_netdev_add(): the QMP front-end for netdev_add(). This is
    coded by hand and not auto-generated (gen=no in the schema). The
    reason for this it's a lot easier and simpler to with QemuOpts
    this way

 3. hmp_netdev_add(): HMP front-end.

This design was suggested by Paolo Bonzini.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-By: Laszlo Ersek <lersek@redhat.com>