net/virtio: rationalize setting of Rx/Tx handlers

net/virtio: rationalize setting of Rx/Tx handlers

The selection of Rx/Tx handlers is done at several places,
group them in one function set_rxtx_funcs().

The update of hw->use_simple_rxtx is also rationalized:
- initialized to 1 (prefer simple path)
- in dev configure or rx/tx queue setup, if something prevents from
  using the simple path, change it to 0.
- in dev start, set the handlers according to hw->use_simple_rxtx.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>