net/virtio: move device initialization in a function

net/virtio: move device initialization in a function

Move all code related to device initialization in a new function
virtio_init_device().

This commit brings no functional change, it prepares the next commits
that will add the offload support. For that, it will be needed to
reinitialize the device from ethdev->configure(), using this new
function.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>